Declarations and Definitions (C++)

A C++ program consists of various entities such as variables, functions, types, and namespaces. Each of these entities must be declared before they can be used. A declaration specifies a unique name for the entity, along with information about its type and other characteristics. In C++ the point at which a name is declared is the point at which it becomes visible to the compiler. You cannot refer to a function or class that is declared at some later point in the compilation unit. Variables should be declared as close as possible before the point at which they are used.

The following example shows some declarations:

#include <string>

void f(); // forward declaration

int main()
{
    const double pi = 3.14; //OK
    int i = f(2); //OK. f is forward-declared
    std::string str; // OK std::string is declared in <string> header
    C obj; // error! C not yet declared.
    j = 0; // error! No type specified.
    auto k = 0; // OK. type inferred as int by compiler.
}

int f(int i)
{
    return i + 42;
}

namespace N {
   class C{/*...*/};
}

On line 5, the main function is declared. On line 7, a const variable named pi is declared and initialized. On line 8, an integer i is declared and initialized with the value produced by the function f. The name f is visible to the compiler because of the forward declaration on line 3.

In line 9, a variable named obj of type C is declared. However, this declaration raises an error because C is not declared until later in the program, and is not forward-declared. To fix the error, you can either move the entire definition of C before main or else add a forward-declaration for it. This behavior is different from other languages such as C#, in which functions and classes can be used before their point of declaration in a source file.

In line 10, a variable named str of type std::string is declared. The name std::string is visible because it is introduced in the string header file which is merged into the source file in line 1. std is the namespace in which the string class is declared.

In line 11, an error is raised because the name j has not been declared. A declaration must provide a type, unlike other languages such as javaScript. In line 12, the auto keyword is used, which tells the compiler to infer the type of k based on the value that it is initialized with. The compiler in this case chooses int for the type.

Declaration scope

The name that is introduced by a declaration is valid within the scope where the declaration occurs. In the previous example, the variables that are declared inside the main function are local variables. You could declare another variable named i outside of main, at global scope, and it would be a completely separate entity. However, such duplication of names can lead to programmer confusion and errors, and should be avoided. In line 21, the class C is declared in the scope of the namespace N. The use of namespaces helps to avoid name collisions. Most C++ Standard Library names are declared within the std namespace. For more information about how scope rules interact with declarations, see Scope.

Definitions

Some entities, including functions, classes, enums, and constant variables, must be defined in addition to being declared. A definition provides the compiler with all the information it needs to generate machine code when the entity is used later in the program. In the previous example, line 3 contains a declaration for the function f but the definition for the function is provided in lines 15 through 18. On line 21, the class C is both declared and defined (although as defined the class doesn't do anything). A constant variable must be defined, in other words assigned a value, in the same statement in which it is declared. A declaration of a built-in type such as int is automatically a definition because the compiler knows how much space to allocate for it.

The following example shows declarations that are also definitions:

// Declare and define int variables i and j.
int i;
int j = 10;

// Declare enumeration suits.
enum suits { Spades = 1, Clubs, Hearts, Diamonds };

// Declare class CheckBox.
class CheckBox : public Control
{
public:
    Boolean IsChecked();
    virtual int     ChangeState() = 0;
};

Here are some declarations that are not definitions:

extern int i;
char *strchr( const char *Str, const char Target );

Typedefs and using statements

In older versions of C++, the typedef keyword is used to declare a new name that is an alias for another name. For example the type std::string is another name for std::basic_string<char>. It should be obvious why programmers use the typedef name and not the actual name. In modern C++, the using keyword is preferred over typedef, but the idea is the same: a new name is declared for an entity which is already declared and defined.

Static class members

Because static class data members are discrete variables shared by all objects of the class, they must be defined and initialized outside the class definition. (For more information, see Classes.)

extern declarations

A C++ program might contain more than one compilation unit. To declare an entity that is defined in a separate compilation unit, use the extern keyword. The information in the declaration is sufficient for the compiler, but if the definition of the entity cannot be found in the linking step, then the linker will raise an error.

See also

Basic Concepts