Question

I saw someone’s C++ code has function declaration like below:

void information_log( const char* fmt , ...)

or catch block like

catch(...)
{
}

What does “…” mean?

Answer 1

The ellipsis ..., in a function prototype, is used to denote the function as variadic. That is, it enables a variable number of arguments to be passed into the function. In this form, a function must define some way for the user to specify exactly how many arguments they presented, since the variadic library functions in C++ can’t determine this information dynamically.

For example, the stdio function printf is one such function with the prototype:

int printf(const char *format, ...);

Presumably, from the similarities between the two prototypes, the information_log function you describe is designed to mirror much of printf‘s functionality and perhaps even internally uses printf, or one of its cousins.

The following is an example of how to implement a variadic function:

// cstdarg provides access to the arguments passed to the ellipsis
#include <cstdarg> // or (#include <stdarg.h>)
#include <cstdio>
#include <cstring>

// Concatenates as many strings as are present
void concatenate(char ** out, int num_str, ...)
{
    // Store where the arguments are in memory
    va_list args;

    // Find the first variadic argument, relative to the last named argument
    va_start(args, num_str);

    int out_len = 0;
    int * lengths = new int[num_str];
    char ** strings = new char*[num_str];

    // Extract the strings from the variadic argument list
    for(int i = 0; i < num_str; i++)
    {
        // Specify the position in the argument list and the type
        // Note: You must know the type, stdarg can't detect it for you
        strings[i] = va_arg(args, char *);
        lengths[i] = strlen(strings[i]);
        out_len += lengths[i];
    }

    // Concatenate the strings
    int dest_cursor = 0;
    (*out) = new char[out_len + 1];
    for(int i = 0; i < num_str; i++)
    {
        strncpy( (*out) + dest_cursor, strings[i], lengths[i]);
        dest_cursor += lengths[i];
    }
    (*out)[dest_cursor] = '\0';

    // Clean up
    delete [] strings;
    delete [] lengths;
    va_end(args);
}

int main()
{
    char * output = NULL;

    // Call our function and print the result
    concatenate(&output, 5, "The ", "quick", " brown ", "fox ", "jumps!\n");
    printf("%s", output);

    delete [] output;
    return 0;
}