Pass variable “name” in C++

Question

I currently use the following template simply as a way to check for NULL pointer and if NULL then print out an error message to a log file and then return false.

template< typename T >
static bool isnull(T * t, std::string name = "")
{
    _ASSERTE( t != 0 );
    if( !t )
    {
        if( !(name.length()) ) name = "pointer";
        PANTHEIOS_TRACE_ERROR(name + " is NULL");
        return false;
    }
    return true;
}

I currently call this as follows:

if( !(isnull(dim, BOOST_STRINGIZE(dim))) ) return false;

If you notice I need to pass in the "name" of the pointer-variable that I want to print to the log file, as the 2nd parameter. I am currently using BOOST_STRINGIZE that simply converts any text inside the parentheses to a string.

The following are the disadvantages of my template implementation (for my usage at least)

• Anyone could pass in anything as parameter to BOOST_STRINGIZE to print out in log file - since the 2 parameters are not related in anyway - so I would not necessarily see the "variable name" that is actually NULL
• We have to remember to pass in the 2nd parameter, else useless.

Is there anyway I can have the "name" of that 1st variable be automatically determined, so that I can omit passing it in, as the 2nd parameter, with every call?

Answer 1

You could put it all in one macro:

#define IS_NULL(name_) isnull(name_, #name_)

Note that BOOST_STRINGIZE expands its argument if its a macro, which may or may not be what you want:

#define X(x_) std::cout << BOOST_STRINGIZE(x_) << " = " << x_ << std::endl;
X(NULL); // prints: "0 = 0"

Answer 2

当然，为什么不：

#define new_isnull(x)  isnull(x, BOOST_STRINGIZE(x))

Answer 3

The only way to do anything lexically like this is with macros. If you always want the correct printout, your best option is to wrap the whole statement in a macro:

//if( !(isnull(dim, BOOST_STRINGIZE(dim))) ) return false;
#define ISNULL(a) isnull((a), #a)
if (!ISNULL(dim)) return false;

Note that, as always, macros have a number of disadvantages associated with them.