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2.6 Object DeclarationsAn object in C++ is a region of storage with a type, a value, and possibly a name. In traditional object-oriented programming, "object" means an instance of a class, but in C++ the definition is slightly broader to include instances of any data type. An object (variable or constant) declaration has two parts: a series of specifiers and a list of comma-separated declarators. Each declarator has a name and an optional initializer. 2.6.1 SpecifiersEach declaration begins with a series of specifiers. The series can contain a storage class, const and volatile qualifiers, and the object's type, in any order. 2.6.1.1 Storage class specifiersA storage class specifier can specify scope linkage and lifetime. The storage class is optional. For function parameters and local variables in a function, the default storage class specifier is auto. For declarations at namespace scope, the default is usually an object with static lifetime and external linkage. C++ has no explicit storage class for such a declaration. (See Section 2.6.4 later in this chapter and Section 2.4 earlier in this chapter for more information.) If you use a storage class specifier, you must choose only one of the following:
2.6.1.2 const and volatile qualifiersThe const and volatile specifiers are optional. You can use either one, neither, or both in any order. The const and volatile keywords can be used in other parts of a declaration, so they are often referred to by the more general term qualifiers; for brevity, they are often referred to as cv-qualifiers.
2.6.1.3 Type specifiersEvery object must have a type in the form of one or more type specifiers. The type specifiers can be any of the following:
Enumerated and fundamental types are described earlier in this chapter in Section 2.5. Class types are covered in Chapter 6. The typename keyword is covered in Chapter 7. 2.6.1.4 Using specifiersSpecifiers can appear in any order, but the convention is to list the storage class first, followed by the type specifiers, followed by cv-qualifiers. extern long int const mask; // Conventional int extern const long mask; // Valid, but strange Many programmers prefer a different order: storage class, cv-qualifiers, type specifiers. More and more are learning to put the cv-qualifiers last, though. See the examples under Section 2.6.2.2 later in this chapter to find out why. The convention for types that require multiple keywords is to place the base type last and the modifiers first: unsigned long int x; // Conventional int unsigned long y; // Valid, but strange long double a; // Conventional double long b; // Valid, but strange You can define a class or enumerated type in the same declaration as an object declaration: enum color { red, black } node_color;
However, the custom is to define the type separately, then use the type name in a separate object declaration: enum color { red, black };
color node_color;
2.6.2 DeclaratorsA declarator declares a single name within a declaration. In a declaration, the initial specifiers apply to all the declarators in the declaration, but each declarator's modifiers apply only to that declarator. (See Section 2.6.2.2 in this section for examples of where this distinction is crucial.) A declarator contains the name being declared, additional type information (for pointers, references, and arrays), and an optional initializer. Use commas to separate multiple declarators in a declaration. For example: int plain_int, array_of_int[42], *pointer_to_int; 2.6.2.1 ArraysAn array is declared with a constant size specified in square brackets. The array size is fixed for the lifetime of the object and cannot change. (For an array-like container whose size can change at runtime, see <vector> in Chapter 13.) To declare a multidimensional array, use a separate set of square brackets for each dimension: int point[2]; double matrix[3][4]; // A 3 x 4 matrix You can omit the array size if there is an initializer; the number of initial values determines the size. In a multidimensional array, you can omit only the first (leftmost) size: int data[] = { 42, 10, 3, 4 }; // data[4]
int identity[][3] = { { 1,0,0 }, {0,1,0}, {0,0,1} }; // identity[3][3]
char str[] = "hello"; // str[6], with trailing \0
In a multidimensional array, all elements are stored contiguously, with the rightmost index varying the fastest (usually called row major order). When a function parameter is an array, the array's size is ignored, and the type is actually a pointer type, which is the subject of the next section. For a multidimensional array used as a function parameter, the first dimension is ignored, so the type is a pointer to an array. Because the first dimension is ignored in a function parameter, it is usually omitted, leaving empty square brackets: long sum(long data[], size_t n); double chi_sq(double stat[][2]); 2.6.2.2 PointersA pointer object stores the address of another object. A pointer is declared with a leading asterisk (*), optionally followed by cv-qualifiers, then the object name, and finally an optional initializer. When reading and writing pointer declarations, be sure to keep track of the cv-qualifiers. The cv-qualifiers in the declarator apply to the pointer object, and the cv-qualifiers in the declaration's specifiers apply to the type of the pointer's target. For example, in the following declaration, the const is in the specifier, so the pointer p is a pointer to a const int. The pointer object is modifiable, but you cannot change the int that it points to. This kind of pointer is usually called a pointer to const. int i, j; int const *p = &i; p = &j; // OK *p = 42; // Error When the cv-qualifier is part of the declarator, it modifies the pointer object. Thus, in the following example, the pointer p is const and hence not modifiable, but it points to a plain int, which can be modified. This kind of pointer is usually called a const pointer. int i, j; int * const p = &i; p = &j; // Error *p = 42; // OK You can have pointers to pointers, as deep as you want, in which each level of pointer has its own cv-qualifiers. The easiest way to read a complicated pointer declaration is to find the declarator, work your way from the inside to the outside, and then from right to left. In this situation, it is best to put cv-qualifiers after the type specifiers. For example: int x; int *p; // Pointer to int int * const cp = &x; // const pointer to int int const * pc; // Pointer to const int int const * const cpc = &x; // const pointer to const int int *pa[10]; // Array of 10 pointers to int int **pp; // Pointer to pointer to int When a function parameter is declared with an array type, the actual type is a pointer, and at runtime the address of the first element of the array is passed to the function. You can use array syntax, but the size is ignored. For example, the following two declarations mean the same thing: int sum(int data[], size_t n); int sum(int *data, size_t n); When using array notation for a function parameter, you can omit only the first dimension. For example, the following is valid: void transpose(double matrix[][3]); but the following is not valid. If the compiler does not know the number of columns, it does not know how to lay out the memory for matrix or compute array indices. void transpose(double matrix[][]); A useful convention is to use array syntax when declaring parameters that are used in an array-like fashion—that is, the parameter itself does not change, or it is dereferenced with the [] operator. Use pointer syntax for parameters that are used in pointer-like fashion—that is, the parameter value changes, or it is dereferenced with the unary * operator. 2.6.2.3 Function pointersA function pointer is declared with an asterisk (*) and the function signature (parameter types and optional names). The declaration's specifiers form the function's return type. The name and asterisk must be enclosed in parentheses, so the asterisk is not interpreted as part of the return type. An optional exception specification can follow the signature. See Chapter 5 for more information about function signatures and exception specifications. void (*fp)(int); // fp is pointer to function that takes an int parameter
// and returns void.
void print(int);
fp = print;
A declaration of an object with a function pointer type can be hard to read, so typically you declare the type separately with a typedef declaration, and then declare the object using the typedef name: typedef void (*Function)(int); Function fp; fp = print; Example 2-11 shows a declaration of an array of 10 function pointers, in which the functions return int* and take two parameters: a function pointer (taking an int* and returning an int*) and an integer. The declaration is almost unreadable without using typedef declarations for each part of the puzzle. Example 2-11. Simplifying function pointer declarations with typedef// Array of 10 function pointers int* (*fp[10])(int*(*)(int*), int); // Declare a type for pointer to int. typedef int* int_ptr; // Declare a function pointer type for a function that takes an int_ptr parameter // and returns an int_ptr. typedef int_ptr (*int_ptr_func)(int_ptr); // Declare a function pointer type for a function that returns int_ptr and takes // two parameters: the first of type int_ptr and the second of type int. typedef int_ptr (*func_ptr)(int_ptr_func, int); // Declare an array of 10 func_ptrs. func_ptr fp[10]; 2.6.2.4 Member pointersPointers to members (data and functions) work differently from other pointers. The syntax for declaring a pointer to a nonstatic data member or a nonstatic member function requires a class name and scope operator before the asterisk. Pointers to members can never be cast to ordinary pointers, and vice versa. You cannot declare a reference to a member. (See Chapter 3 for information about expressions that dereference pointers to members.) A pointer to a static member is an ordinary pointer, not a member pointer. The following are some simple examples of member pointers: struct simple {
int data;
int func(int);
};
int simple::* p = &simple::data;
int (simple::*fp)(int) = &simple::func;
simple s;
s.*p = (s.*fp)(42);
2.6.2.5 ReferencesA reference is a synonym for an object or function. A reference is declared just like a pointer, but with an ampersand (&) instead of an asterisk (*). A local or global reference declaration must have an initializer that specifies the target of the reference. Data members and function parameters, however, do not have initializers. You cannot declare a reference of a reference, a reference to a class member, a pointer to a reference, an array of references, or a cv-qualified reference. For example: int x; int &r = x; // Reference to int int& const rc = x; // Error: no cv qualified references int &&rr; // Error: no reference of reference int& ra[10]; // Error: no arrays of reference int*&* rp = &r; // Error: no pointer to reference int* p = &x; // Pointer to int int*&* pr = p; // OK: reference to pointer A reference, unlike a pointer, cannot be made to refer to a different object at runtime. Assignments to a reference are just like assignments to the referenced object. Because a reference cannot have cv-qualifiers, there is no such thing as a const reference. Instead, a reference to const is often called a const reference for the sake of brevity. References are often used to bind names to temporary objects, implement call-by-reference for function parameters, and optimize call-by-value for large function parameters. The divide function in Example 2-12 demonstrates the first two uses. The standard library has the div function, which divides two integers and returns the quotient and remainder in a struct. Instead of copying the structure to a local object, divide binds the return value to a reference to const, thereby avoiding an unnecessary copy of the return value. Furthermore, suppose that you would rather have divide return the results as arguments. The function parameters quo and rem are references; when the divide function is called, they are bound to the function arguments, q and r, in main. When divide assigns to quo, it actually stores the value in q, so when divide returns, main has the quotient and remainder. Example 2-12. Returning results in function arguments#include <cstdlib>
#include <iostream>
#include <ostream>
void divide(long num, long den, long& quo, long& rem)
{
const std::ldiv_t& result = std::div(num, den);
quo = result.quot;
rem = result.rem;
}
int main( )
{
long q, r;
divide(42, 5, q, r);
std::cout << q << " remainder " << r << '\n';
}
The other common use of references is to use a const reference for function parameters, especially for large objects. Function arguments are passed by value in C++, which requires copying the arguments. The copy operation can be costly for a large object, so passing a reference to a large object yields better performance than passing the large object itself. The reference parameter is bound to the actual argument, avoiding the unnecessary copy. If the function modifies the object, it would violate the call-by-value convention, so you should declare the reference const, which prevents the function from modifying the object. In this way, call-by-value semantics are preserved, and the performance of call-by-reference is improved. The standard library often uses this idiom. For example, operator<< for std::string uses a const reference to the string to avoid making unnecessary copies of the string. (See <string> in Chapter 13 for details.) If a function parameter is a non-const reference, the argument must be an lvalue. A const reference, however, can bind to an rvalue, which permits temporary objects to be passed to the function, which is another characteristic of call-by-value. (See Chapter 3 for the definitions of "lvalue" and "rvalue.") A reference must be initialized so it refers to an object. If a data member is a reference, it must be initialized in the constructor's initializer list (see Chapter 6). Function parameters that are references are initialized in the function call, binding each reference parameter to its corresponding actual argument. All other reference definitions must have an initializer. (An extern declaration is not a definition, so it doesn't take an initializer.) A const reference can be initialized to refer to a temporary object. For example, if a function takes a const reference to a float as a parameter, you can pass an integer as an argument. The compiler converts the integer to float, saves the float value as an unnamed temporary object, and passes that temporary object as the function argument. The const reference is initialized to refer to the temporary object. After the function returns, the temporary object is destroyed: void do_stuff(const float& f);
do_stuff(42);
// Equivalent to:
{
const float unnamed = 42;
do_stuff(unnamed);
}
The restrictions on a reference, especially to a reference of a reference, pose an additional challenge for template authors. For example, you cannot store references in a container because a number of container functions explicitly declare their parameters as references to the container's value type. (Try using std::vector<int&> with your compiler, and see what happens. You should see a lot of error messages.) Instead, you can write a wrapper template, call it rvector<typename T>, and specialize the template (rvector<T&>) so references are stored as pointers, but all the access functions hide the differences. This approach requires you to duplicate the entire template, which is tedious. Instead, you can encapsulate the specialization in a traits template called Ref<> (refer to Chapter 7 for more information about templates and specializations, and to Chapter 8 for more information about traits), as shown in Example 2-13. Example 2-13. Encapsulating reference traits// Ref type trait encapsulates reference type, and mapping to and from the type
// for use in a container.
template<typename T>
struct Ref {
typedef T value_type;
typedef T& reference;
typedef const T& const_reference;
typedef T* pointer;
typedef const T* const_pointer;
typedef T container_type;
static reference from_container(reference x) { return x; }
static const_reference from_container(const_reference x)
{ return x; }
static reference to_container(reference x) { return x; }
};
template<typename T>
struct Ref<T&> {
typedef T value_type;
typedef T& reference;
typedef const T& const_reference;
typedef T* pointer;
typedef const T* const_pointer;
typedef T* container_type;
static reference from_container(pointer x) { return *x; }
static const_reference from_container(const_pointer x)
{ return *x; }
static pointer to_container(reference x) { return &x; }
};
// rvector<> is similar to vector<>, but allows references by storing references
// as pointers.
template<typename T, typename A=std::allocator<T> >
class rvector {
typedef typename Ref<T>::container_type container_type;
typedef typename std::vector<container_type> vector_type;
public:
typedef typename Ref<T>::value_type value_type;
typedef typename Ref<T>::reference reference;
typedef typename Ref<T>::const_reference const_reference;
typedef typename vector_type::size_type size_type;
. . . // Other typedefs are similar.
class iterator { ... }; // Wraps a vector<>::iterator
class const_iterator { ... };
. . . // Constructors pass arguments to v.
iterator begin( ) { return iterator(v.begin( )); }
iterator end( ) { return iterator(v.end( )); }
void push_back(typename Ref<T>::reference x) {
v.push_back(Ref<T>::to_container(x));
}
reference at(size_type n) {
return Ref<T>::from_container(v.at(n));
}
reference front( ) {
return Ref<T>::from_container(v.front( ));
}
const_reference front( ) const {
return Ref<T>::from_container(v.front( ));
}
. . . // Other members are similar.
private:
vector_type v;
};
2.6.3 InitializersAn initializer supplies an initial value for an object being declared. You must supply an initializer for the definition of a reference or const object. An initializer is optional for other object definitions. An initializer is not allowed for most data members within a class definition, but an exception is made for static const data members of integral or enumerated type. Initializers are also not allowed for extern declarations and function parameters. (Default arguments for function parameters can look like initializers. See Chapter 5 for details.) The two forms of initializers are assignment-like and function-like. (In the C++ standard, assignment-like is called copy initialization, and function-like is called direct initialization.) An assignment-like initializer starts with an equal sign, which is followed by an expression or a list of comma-separated expressions in curly braces. A function-like initializer is a list of one or more comma-separated expressions in parentheses. Note that these initializers look like assignment statements or function calls, respectively, but they are not. They are initializers. The difference is particularly important for classes (see Chapter 6). The following are some examples of initializers: int x = 42; // Initializes x with the value 42
int y(42); // Initializes y with the value 42
int z = { 42 }; // Initializes z with the value 42
int w[4] = { 1, 2, 3, 4 }; // Initializes an array
std::complex<double> c(2.0, 3.0); // Calls complex constructor
When initializing a scalar value, the form is irrelevant. The initial value is converted to the desired type using the usual conversion rules (as described in Chapter 3). Without an initializer, all non-POD class-type objects are initialized by calling their default constructors. (See Chapter 6 for more information about POD and non-POD classes.) All other objects with static lifetimes are initialized to 0; objects with automatic lifetimes are left uninitialized. (See Section 2.6.4 later in this chapter.) An uninitialized reference or const object is an error. 2.6.3.1 Function-like initializersYou must use a function-like initializer when constructing an object whose constructor takes two or more arguments, or when calling an explicit constructor. The usual rules for resolving overloaded functions apply to the choice of overloaded constructors. (See Chapter 5 for more information about overloading and Chapter 6 for more information about constructors.) For example: struct point {
point(int x, int y);
explicit point(int x);
point( );
...
};
point p1(42, 10); // Invokes point::point(int x, int y);
point p2(24); // Invokes point::point(int x);
point p3; // Invokes point::point( );
Empty parentheses cannot be used as an initializer in an object's declaration, but can be used in other initialization contexts (namely, a constructor initializer list or as a value in an expression). If the type is a class type, the default constructor is called; otherwise, the object is initialized to 0. Example 2-14 shows an empty initializer. No matter what type T is, the wrapper<> template can rely on T( ) to be a meaningful default value. Example 2-14. An empty initializertemplate<typename T>
struct wrapper {
wrapper( ) : value_(T( )) {}
explicit wrapper(const T& v) : value_(v) {}
private:
T value_;
};
wrapper<int> i; // Initializes i with int( ), or zero
enum color { black, red, green, blue };
wrapper<color> c; // Initializes c with color( ), or black
wrapper<bool> b; // Initializes b with bool( ), or false
wrapper<point> p; // Initializes p with point( )
2.6.3.2 Assignment-like initializersIn an assignment-like initializer, if the object is of class type, the value to the right of the equal sign is converted to a temporary object of the desired type, and the first object is constructed by calling its copy constructor. The generic term for an array or simple class is aggregate because it aggregates multiple values into a single object. "Simple" in this case means the class does not have any of the following:
To initialize an aggregate, you can supply multiple values in curly braces, as described in the following sections. A POD object is a special kind of an aggregate. (See Section 2.5.3 earlier in this chapter for more information about POD types; see also Chapter 6 for information about POD classes.) To initialize an aggregate of class type, supply an initial value for each nonstatic data member separated by commas and enclosed in curly braces. For nested objects, use nested curly braces. Values are associated with members in the order of the members' declarations. More values than members results in an error. If there are fewer values than members, the members without values are initialized by calling each member's default constructor or initializing the members to 0. An initializer list can be empty, which means all members are initialized to their defaults, which is different from omitting the initializer entirely. The latter causes all members to be left uninitialized. The following example shows several different initializers for class-type aggregates: struct point { double x, y, z; };
point origin = { }; // All members initialized to 0.0
point unknown; // Uninitialized, value is not known
point pt = { 1, 2, 3 }; // pt.x==1.0, pt.y==2.0, pt.z==3.0
struct line { point p1, p2; };
line vec = { { }, { 1 } }; // vec.p1 is all zero.
// vec.p2.x==1.0, vec.p2.y==0.0, vec.p2.z==0.0
Only the first member of a union can be initialized: union u { int value; unsigned char bytes[sizeof(int)]; };
u x = 42; // Initializes x.value
2.6.3.3 Initializing arraysInitialize elements of an array with values separated by commas and enclosed in curly braces. Multidimensional arrays can be initialized by nesting sets of curly braces. An error results if there are more values than elements in the array; if an initializer has fewer values than elements in the array, the remaining elements in the array are initialized to zero values (default constructors or 0). If the declarator omits the array size, the size is determined by counting the number of values in the initializer. An array initializer can be empty, which forces all elements to be initialized to 0. If the initializer is empty, the array size must be specified. Omitting the initializer entirely causes all elements of the array to be uninitialized (except non-POD types, which are initialized with their default constructors). In the following example the size of vec is set to 3 because its initializer contains three elements, and the elements of zero are initialized to 0s because an empty initializer is used: int vec[] = { 1, 2, 3 }; // Array of three elements
// vec[0]==1 ... vec[2]==3
int zero[4] = { }; // Initialize to all zeros.
When initializing a multidimensional array, you can flatten the curly braces and initialize elements of the array in row major order (last index varies the fastest). For example, both id1 and id2 end up having the same values in their corresponding elements: // Initialize id1 and id2 to the identity matrix.
int id1[3][3] = { { 1 }, { 0, 1 }, { 0, 0, 1 } };
int id2[3][3] = { 1, 0, 0, 0, 1, 0, 0, 0, 1 };
An array of char or wchar_t is special because you can initialize such arrays with a string literal. Remember that every string literal has an implicit null character at the end. For example, the following two char declarations are equivalent, as are the two wchar_t declarations: // The following two declarations are equivalent.
char str1[] = "Hello";
char str2[] = { 'H', 'e', 'l', 'l', 'o', '\0' };
wchar_t ws1[] = L"Hello";
wchar_t ws2[] = { L'H', L'e', L'l', L'l', L'o', L'\0' };
The last expression in an initializer list can be followed by a comma. This is convenient when you are maintaining software and find that you often need to change the order of items in an initializer list. You don't need to treat the last element differently from the other elements. const std::string keywords[] = {
"and",
"asm",
...
"while",
"xor",
};
Because the last item has a trailing comma, you can easily select the entire line containing "xor" and move it to a different location in the list, and you don't need to worry about fixing up the commas afterward. 2.6.3.4 Initializing scalarsYou can initialize any scalar object with a single value in curly braces, but you cannot omit the value the way you can with a single-element array: int x = { 42 };
2.6.4 Object LifetimeEvery object has a lifetime, that is, the duration from when the memory for the object is allocated and the object is initialized to when the object is destroyed and the memory is released. Object lifetimes fall into three categories:
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