Namespaces can also be customized

Definition of namespaces

namespaces, in addition to the system-defined namespaces, can be defined by themselves, defining namespaces with the keyword "namespace", using the namespace with the symbol "::" specified.

A variable or function that does not specify a namespace is a variable or function under the current namespace.

If you do not define a namespace, all of them belong to the global namespace.

The same namespace can be defined multiple times.

namespace definitions and usage examples:

#include <iostream>

using namespace Std;

Namespace NA

{

void print (int n) {//is the same as the function name of the namespace NB

cout << "na::p rint:" << n << Endl;

}

}

Namespace NB

{

void print (int n) {//is the same as the function name of namespace NA

cout << "NB::p rint:" << n << Endl;

}

}

Namespace na//the same namespace can be defined multiple times

{

void Print2 (int a, int b) {

Print (a);

Print (b);

}

}

int main ()

{

NA::p rint (3); Specify the print function under NA

NB::p rint (4); Specify the print function under NA

NA::p rint2 (5, 6); Specify the Print2 function under NA

return 0;

}

Operation Result:

NA::p rint:3

NB::p rint:4

NA::p rint:5

NA::p rint:6

Use of using

Using the "using namespace" allows you to specify the namespace at a later time, which is equivalent to the Import function in Java. However, when using the "using namespace" multiple times, the namespaces that are specified are valid and conflict-prone.

#include <iostream>

using namespace Std;

Namespace NA

{

void print (int a) {

cout << "na::p rint:" << a << Endl;

}

}

Namespace NB

{

using namespace na;

void Print2 (int a, int b) {//This is the function of NB itself

Print (a); Functions of NA

Print (b); Functions of NA

}

void print (int n) {//This is also the function of NB itself

cout << "NB::p rint:" << n << Endl;

}

}

Namespace NC

{

Using NB::p rint2; Specifies that only NB's Print2 () function is used

void print (int a, int b, int c) {

Print2 (A, b); This is the function of NB.

NB::p rint (c); The using is not specified, so you must use NB:: To specify

}

}

int main ()

{

using namespace na;

Print (2); The print () function of NA

using namespace NB; When used in this way, NA and NB are available simultaneously

Print (3); The compilation system is not sure which of Na and NB to call, because the function name and parameters are the same

using namespace NC; Na and NC are also valid when used in this way

Print (4, 5, 6); The parameters of the NC print () function are different from the parameters in Na, so there is no error here.

Using NB::p rint; Specifies that the following print () function is a function of NB

Print (7);

return 0;

}

Operation Result:

NA::p rint:2

NA::p rint:4

NA::p rint:5

NB::p rint:6

NB::p rint:7

Specify namespaces with aliases

Namespaces can be nested layers, specify when you have to specify a layer, so it is inconvenient, the general use of aliases instead of more convenient.

#include <iostream>

using namespace Std;

Namespace Na {

Namespace NB {

Namespace NC {

int sum (int a, int b)

{

return a + B;

}

}

}

}

namespace A = Na::nb::nc;

int main ()

{

cout << Na::nb::nc::sum (5, one) << Endl;

cout << A::sum (6, a) << Endl;

return 0;

}

Operation Result:

16

18

namespaces without a name

Namespace names can also be omitted, known as "Nameless namespace", nameless namespace is often used, its role is "internal can be referenced, and external cannot reference."

#include <iostream>

using namespace Std;

Namespace Na {

namespace {//unknown namespace

int sum (int a, int b)

{

return a + B;

}

}

int calc (int x, int y)

{

return sum (x, y); Inside a variable or function that can invoke the nameless namespace

}

}

int main ()

{

cout << Na::sum (5, one) << Endl; Because the name inside is missing, the external cannot be called

cout << Na::calc (6, a) << Endl;

return 0;

}

Operation Result:

18

Namespaces can also be customized