Namespace-what you know and what you don't know

Namespace is a feature of C ++. It can provide solutions for name conflicts in large projects. However, manyProgramStaff ignored its use.

Here are a few examples to illustrate some of its usage.

1. Using declarations

It provides a local alias for the objects declared in the namespace;

Namespace A <br/>{< br/> int F (INT); <br/> int I; <br/>}// note, the semicolon is not required here </P> <p> using a: F; // introduce the f function in a; </P> <p> int F (INT ); // The global function </P> <p> int main () <br/> {<br/> F (1); // The ambiguity is incorrect! <Br/> int I; // defines the local variable <br/> using a: I; // error! Just like writing another line of int I; <br/>}

Using declarations only references the names in the previously defined namespace and is invalid for subsequent declarations:

Namespace A <br/>{< br/> Class X {}; // #1 <br/> int y (); // #2 <br/> int F (double); // #3 <br/>}</P> <p> using a: X; // only #1 <br/> using a: Y; // only #2 <br/> using a: F; // only #3 </P> <p> namespace A <br/>{< br/> class y {}; // #4 <br/> int F (INT); // #5 <br/>}</P> <p> int main () <br/> {<br/> X; // OK <br/> Y; // error, #4 invisible <br/> F (1 ); // call #3! <Br/>}</P> <p>

 

2. Using directives

The name of the entire namespace will be introduced and can be referenced to the namespace name defined later in this instruction!

Namespace A <br/>{< br/> Class X {}; <br/> int F (double ); <br/>}</P> <p> using namespace A; </P> <p> void F () <br/>{< br/> X; // A: x <br/> Y; // error! <Br/> F (1); // OK: int F (double) <br/>}</P> <p> namespace A <br/>{< br/> class y {}; <br/> int F (INT ); <br/>}</P> <p> int main () <br/>{< br/> X; // :: x <br/> Y; // A: y <br/> F (1); // OK: int F (INT) <br/>}< br/>

 

Therefore, do not use using in the header file to avoid unexpected conflicts. Using in CPP cannot appear before include.