We have seen that the definition of a namespace member can appear within the namespace definition itself For example, the class matrix and the constant pi are defined within the definition of the nested namespace MatrixLib, whereas the definitions of the functions operator+() and inverse() are provided at some later point in the program:

// ---- primerh ---namespace cplusplus_primer { // first nested namespace: // defines the matrix portion of the library namespace MatrixLib { class matrix { /* */ }; const double pi = 31416; matrix operator+ ( const matrix &m1, const matrix &m2 ); void inverse( matrix & ); // } }

It is also possible to define any namespace member outside its namespace definition In such a case, the name of the namespace member must be qualified by the names of its enclosing namespaces For example, the function operator+() can be defined in global scope as follows:

// ---- primerC ---#include "primerh" // global scope definition cplusplus_primer::MatrixLib::matrix cplusplus_primer::MatrixLib::operator+ ( const matrix& m1, const matrix &m2 ) { /* */ }

In this definition, the name operator+() is qualified by the names of namespaces cplusplus_primer and MatrixLib However, look at the use of the type matrix in the parameter list of operator+() The name used is not qualified with the nested namespace name cplusplus_primer::MatrixLib How can this be The definition of operator+() can use the names of the namespace members in their short form This is because the definition of a namespace member is in the scope of its namespace The members of namespace MatrixLib are considered when the names used in the definition of operator+() are resolved Note, however, that the return type must still be qualified This is because the return type is not in the scope of a function's definition The namespace members can be used in their short forms only following the name of the member:

The definition of operator+() can use the namespace member names in their short form in any declaration or expression within the parameter list or the function body For example, a local declaration within operator+() can create an object of class type matrix as follows:

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// ---- primerC ---#include "primerh" cplusplus_primer::MatrixLib::matrix cplusplus_primer::MatrixLib::operator+ ( const matrix &m1, const matrix &m2 ) { // declares a local variable of type: // cplusplus_primer::MatrixLib::matrix matrix res; // calculate the sum of two matrix objects return res; }

Although a namespace member can be defined outside its namespace definition, there are restrictions on where this definition can appear Only namespaces enclosing the member declaration can contain its definition For example, operator+() can be defined in global scope, in the namespace cplusplus_primer, or in the namespace MatrixLib These are the only three possibilities A definition in namespace MatrixLib would look like this:

// ---- primerC ---#include "primerh" namespace cplusplus_primer { MatrixLib::matrix MatrixLib::operator+ ( const matrix &m1, const matrix &m2 ) { /* */ } }

Note that a namespace member can be defined outside its namespace definition only if the member was previously declared within the namespace definition The definition of operator+() just shown would be in error if it were not preceded by the following declaration that appears in primerh:

namespace cplusplus_primer { namespace MatrixLib { class matrix { /* */ }; // the following declaration cannot be omitted matrix operator+ ( const matrix &m1, const matrix &m2 ); // } }

ODR and Namespace Members As was mentioned earlier, a namespace definition can be discontiguous and can span many files A namespace member can therefore be declared in many files For example:

// primerh namespace cplusplus_primer { // void inverse( matrix & );

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