vsprintf issue

vsprintf_issue.cpp

#include <cstdarg>
#include <string>
#include <iostream>

void debug(std::string c_fmt, ...);

int main() {
	debug("Testing: %d", 12345);  // Testing: -858993460
}

void debug(std::string fmt, ...)
{
    char buf[1024] = {0};
    const char* c_fmt = fmt.c_str();

    va_list ap;
    va_start(ap, c_fmt);
    vsprintf(buf, c_fmt, ap);
    va_end(ap);

    std::cout << buf;
}