c++ forward function call

问题

Is it possible to transfer list of parameters of a function , to another function?

For example in my functionA I want to call my functionB/functionC (depends on the state of execution) with the parameters from the varargs list. Please note, i cannot change functionB/functionC declaration.

int functionA(int a, ...){
    ...
    va_list listPointer;
    va_start( listPointer, a);
    ...
}

int functionB(long b, long c, long d){
    ...
    ...
}

int functionC(long b, int c, int d){
    ...
    ...
}

For this project I use gcc 4.9.1.

What i have tried till now is to pass the void* from the listPointer but it did not work...

Extracting variables from the va_list also will not work because i have like 80 other similair functions which should be called from the functionA , meaning i cannot extract parameters and call by extracted values.

Maybe there is a way to copy memory of the functionA parameters and call functionB/functionC with a pointer to it? Does anyone have an idea of how it would be possible?

回答1:

If you cannot change your functionB, then you have to extract arguments from your functionA va list:

#include <stdarg.h>
#include <stdio.h>

int functionB(long b, long c, long d)
{
    return printf("b: %d, c: %d, d: %d\n", b, c, d);
}

int functionA(int a, ...)
{
    ...
    va_list va;
    va_start(va, a);
    long b = va_arg(va, long);
    long c = va_arg(va, long);
    long d = va_arg(va, long);
    va_end(va);
    return functionB(b, c, d);
}

Maybe there is a way to copy memory of the functionA parameters and call functionB/functionC with a pointer to it? Does anyone have an idea of how it would be possible?

Then it means that you would have to change declaration of your functionB, functionC etc. You might as well then change them to accept va_list instead:

int functionA(int a, va_list args);
int functionC(int c, va_list args);

回答2:

If you have only longs in your va_args that can work.

int functionA(int a, ...){
    va_list listPointer;
    va_start( listPointer, a);
    long b = va_arg(listPointer, long);
    long c = va_arg(listPointer, long);
    long d = va_arg(listPointer, long);
    va_end(listPointer);
    return functionB(b, c, d);
}

回答3:

You can't change the signature of B, but can you change the one of A? If so, this might be a good option:

template <typename... Args>
int functionA(Args&& ... args)
{
    return functionB(std::forward<Args>(args)...);
}

来源：https://stackoverflow.com/questions/43976035/c-forward-function-call

标签

c++

variadic-functions