Tibi Tibi - 28 days ago 10
C++ Question

Is it possible to place a macro in a namespace in c++?

My application uses another output than the standard output for logging information, which is why I wrote my own

Log()
,
Error()
,
Panic()
and
Assert()
functions. To organize things nicely, I enclose all the debugging stuff in a
Debug
namespace.

It would make more sense for the
Assert()
function to also provide a source file and line number, which is only possible using the
__LINE__
and
__FILE__
macros. However, it is pretty unpleasant, inefficient etc... to always have to specify these two parameters.

So this is how my code would look like:

namespace Debug {
void Assert (int condition, std::string message, std::string file, int line);
}


My question is, is it possible to place a macro which includes those two parameters inside the
Debug
namespace? Like this:

namespace Debug {
void Assert_ (int condition, std::string message, std::string file, int line);
#define Assert(a,b) Assert_(a, b, __FILE__, __LINE__)
}

// .... Somewhere where I call the function ....
Debug::Assert (some_condition, "Some_condition should be true");

// Output: Assertion failed on line 10 in file test.cpp:
// Some_condition should be true


Is this valid c++? If not, is there any way of making this work?

Answer

#define is a preprocessor directive. The macros are being replaced before anything else apart from removing comments (which means, before compilation). So at the time macros are replaced, the compiler knows nothing about your namespaces.

As other people state, in your case it will be fine. However, This is how you can get problems:

namespace A
{
 void Assert_ (int condition, std::string message, std::string file, int line)
 {
     std::cout << "A";
 }
   #define Assert(a,b) Assert_(a, b, __FILE__, __LINE__)

}
namespace B
{
 void Assert_ (int condition)
 {
     std::cout << "B";
 }
   #define Assert(a,b) Assert_(a)

}

int main(int argc, char *argv[])
{
    A::Assert(0,"asdasd");
    B::Assert(0,"asdasd");
}

So while it looks the defines are "in the namespaces", the are not, and the last #define will be always be used, which in this case will lead to a compile-time error, because the code in main will be replaced by:

A::Assert(0);
B::Assert(0);

instead of

A::Assert(0,"asdasd", _FILE_, _LINE_);
B::Assert(0);