C++ explanation of two compilation errors related to namespace

Once, in a large project code, I call:

#include <iostream>
using namespace std;
namespace A
{
void fun()
{
 printf("aaa\n");
}
}
namespace B
{
void fun()
{
 printf("bbb\n");
}
}
int main()
{
 fun();
 return 0;
}

Compilation error: error: ‘fun' was not declared in this scope , checked 1, the original is a space in mischief. By the way, why not indent a function in a namespace? I thought about it for a second and understood why the people who were writing the code were doing it.

Here's another mistake I encountered:

#include <iostream>
using namespace std;
namespace A
{
 void fun()
 {
 printf("aaa\n");
 }
}
namespace B
{
 void fun()
 {
 printf("bbb\n");
 }
}
using namespace A;
using namespace B;
int main()
{
 fun();
 return 0;
}

Results: call of overloaded ‘fun()' is ambiguous The mistake is obvious. In practice, it is a common mistake.

conclusion