我们可以在g ++实现中重载sprintf吗？

Question

My friend was assigned to work on an existing C++ project of his company. In the source code he saw something like this:

char* tmp = "882.30";
char buff[32];
sprintf(buff, "%e", tmp);
printf("TEST: %s\n", buff);

When compiling, a warning was raised: warning: format '%e' expects type 'double', but argument 3 has type 'char*' but the output was amazing: 8.823000e+002 . He was curious. How could it be?

To clarify that he created a new small project and he used the same above code snippet and same compiler but it showed very different result: 2.647480e-314 .

The environment: Centos 5.x, gcc 4.4.7.

What could be the reason here? The existing project has overloading function of sprintf or compilers?

Answer 1

That's undefined behavior.

C99 §7.19.6.1/9

If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

That means you are guaranteed to get unportable results/behavior.

Answer 2

You need atof to explicit convert from char * pointer to double

#include <cstdio>
#include <cstdlib>

int main( void )
{
    const char* tmp = "882.30";
    char buff[32];
    sprintf(buff, "%e", atof( tmp )); 
    printf("TEST: %s\n", buff);
    return 0;
}

BTW, you can't override an existing function because that is violation to the One Definition Rule (ODR.)