Question

I was trying to clean up some code that uses char* with std::string and ran into a problem that is illustrated by the following code.

void Foo( int xIn , const std::string & fooIn )
{
    std::cout << "string argument version called \n";
}

void Foo( int xIn ,  bool flagIn = true )
{
    std::cout << "bool argument version called \n";
}

int main()
{
    int x = 1;
    Foo( x , "testing" );
    return 0;
}

When I run the program I get bool argument version called. Is a char* to bool conversion preferred over char* to const std::string& or is Visual Studio 2008 playing tricks on me ?

Answer 1

Surprising as this behaviour is, the compiler is compliant: char* to bool conversion is preferred over the conversion to std::string.

Read more here.

The exact rules are spelled out in the C++ standard. They’re surprisingly complicated, but the following paragraph is crucial here:

C++11 13.3.3.2 Ranking implicit conversion sequences [over.ics.rank]

2
When comparing the basic forms of implicit conversion sequences (as
defined in 13.3.3.1) — a standard conversion sequence (13.3.3.1.1) is
a better conversion sequence than a user-defined conversion sequence
or an ellipsis conversion sequence

char*-to-bool requires a “standard conversion sequence” whereas char*-to-string requires a “user-defined conversion sequence”. Therefore, the former is preferred.