Example of error caused by using directive in namespaces

Question

I'm trying to understand what kind of errors could arise from including using declarations in namespaces. I'm taking into account these links.

I'm trying to create an example where an error is being caused by a name being silently replaced by an header file being loaded before another one, due to usage of the using declaration.

Here I'm defining MyProject::vector:

// base.h
#ifndef BASE_H
#define BASE_H

namespace MyProject
{
    class vector {};
}

#endif

This is the "bad" header: here I'm trying to trick using into shadowing other possible definitions of vector inside MyNamespace:

// x.h
#ifndef X_H
#define X_H

#include <vector>

namespace MyProject
{
    // With this everything compiles with no error!
    //using namespace std;

    // With this compilation breaks!
    using std::vector;
}

#endif

This is the unsuspecting header trying to use MyProject::vector as defined in base.h:

// z.h
#ifndef Z_H
#define Z_H

#include "base.h"

namespace MyProject
{
    void useVector()
    {
        const vector v;
    }
}

#endif

And finally here's the implementation file, including both x.h and z.h:

// main.cpp
// If I swap these two, program compiles!
#include "x.h"
#include "z.h"

int main()
{
    MyProject::useVector();
}

If I include using std::vector in x.h, an actual compilation error happens, telling me that I must specify a template argument when using vector in z.h, because x.h successfully managed to shadow the definition of vector inside MyProject. Is this a good example of why using declarations shouldn't be used in header files, or things go deeper that this, and I'm missing much more?

If I include using namespace std in x.h, however, the shadowing doesn't occur, and the program compiles just fine. Why is that? Shouldn't using namespace std load all names visible under std, including vector, thus shadowing the other one?

Mark B · Accepted Answer · 2016-10-03 14:22:16Z

1

If I include using namespace std in x.h, however, the shadowing doesn't occur, and the program compiles just fine. Why is that?

This much I can answer from 7.3.4/2-3:

First

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive.

Then followed up:

A using-directive does not add any members to the declarative region in which it appears.

So a using-directive (using namespace) only makes the names usable from the target namespace, it does not make them members of it. So any existing member will be preferred over the used namespace member.

answered Oct 3, 2016 at 14:22

Mark B

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1 Comment

swahnee Over a year ago

Thanks! I wasn't aware of this distinction between the two usage of using.

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