Incorrect function call with variable parameters - c++

Question

I am trying to make a nested function call with a variable number of parameters without retrieving them. And I get the wrong result.
Here is my simple c++ program:
extern "C" {
#include <stdio.h>
}
#include <cstdarg>
class ctty {
public:
ctty();
int logger(int prior, const char* format, ...);
private:
};
ctty::ctty(){};
int ctty::logger(int prior, const char* format, ...)
{
va_list ap;
va_start(ap,format);
printf(format, ap);
va_end(ap);
return 0;
}
int main(int argc, char** argv)
{
ctty tty;
tty.logger(0, "Test %d %d %d\n", 7, 5, 5);
return 0;
}
result:
Test -205200 7 5
I expect a result
Test 7 5 5
I don’t understand what am I doing wrong?
Thanks in advance.

Answer 1

You can't directly pass va_list to printf. va_list is a wrapper around the actual list of arguments (whatever its representation is). Although C way to do should be to use vprintf, in C++ there are safer alternatives, like variadic templates allowing to create a safer version of formatted printing, e.g. (a fictional format string for brevity of example):
#include <iostream>
#include <cstdlib>
class ctty {
public:
ctty();
template<typename T, typename... Args>
int logger(int prior, const char* format, T value, Args... args);
private:
void logger(int prior, const char *s);
};
ctty::ctty(){};
void ctty::logger(int prior, const char *s)
{
while (*s) {
if (*s == '%') {
if (*(s + 1) == '%') {
++s;
}
else {
throw std::runtime_error("invalid format string: missing arguments");
}
}
std::cout << *s++;
}
}
template<typename T, typename... Args>
int ctty::logger(int prior, const char* format, T value, Args... args)
{
while (*format) {
if (*format == '%') {
std::cout << value;
logger(prior, format + 1, args...);
return 0;
}
std::cout << *format++;
}
throw std::logic_error("extra arguments provided to logger");
}
int main(int argc, char** argv)
{
ctty tty;
tty.logger(0, "Test % % %\n", 7.55f, "Tada!", 888);
return 0;
}
This part of your code:
extern "C" {
#include <stdio.h>
}
Is technically an undefined behavior, while it may compile and not have adverse effect in some cases, it's not portable. You have to use C++ headers, e.g. <cstdio>