Re: General Allocator Regarding type definitions and void * specialized problem

On 2008-09-07 11:27, PeterAPIIT@gmail.com wrote:


Since you have not quoted the part you are replying to I can only guess
you meant "why can I not use "using namespace" in a header file?" and
the answer is that you can, but it is bad. The reason is simple, every
file that included your header will get the effects of the "using
namespace", consider the following:

------ Test.h -----

#ifndef TEST_H
#define TEST_H

#include <iostream>

using namespace std;

void foo()
{
    std::cout << "foo()\n";
}

#endif

------ Test.cpp -----

#include <vector>

#include "Test.h"

template<typename T>
class vector
{
    T t;
};

int main()
{
    vector<int> v; // Error
}

The problem is that the "using namespace std;" in Test.h" is in effect
in Test.cpp, and vector<int> can now refer to your class or std::vector.
In some cases you might get even worse problems because you do not get
an ambiguity and instead you use some type/function which you did not
intend to use, and the only clue is that the program does not work as it
should.

Therefore you should avoid "using namespace" in headers.

--
Erik Wikstr??m