variadic functions - Variable number of arguments in C++?

Question

How can I write a function that accepts a variable number of arguments? Is this possible, how?

Answer 1

In C++11 you have two new options, as the Variadic functions reference page in the Alternatives section states:

• Variadic templates can also be used to create functions that take variable number of arguments. They are often the better choice because they do not impose restrictions on the types of the arguments, do not perform integral and floating-point promotions, and are type safe. (since C++11)
• If all variable arguments share a common type, a std::initializer_list provides a convenient mechanism (albeit with a different syntax) for accessing variable arguments.

Below is an example showing both alternatives (see it live):

#include <iostream>
#include <string>
#include <initializer_list>

template <typename T>
void func(T t) 
{
    std::cout << t << std::endl ;
}

template<typename T, typename... Args>
void func(T t, Args... args) // recursive variadic function
{
    std::cout << t <<std::endl ;

    func(args...) ;
}

template <class T>
void func2( std::initializer_list<T> list )
{
    for( auto elem : list )
    {
        std::cout << elem << std::endl ;
    }
}

int main()
{
    std::string
        str1( "Hello" ),
        str2( "world" );

    func(1,2.5,'a',str1);

    func2( {10, 20, 30, 40 }) ;
    func2( {str1, str2 } ) ;
} 

If you are using gcc or clang we can use the PRETTY_FUNCTION magic variable to display the type signature of the function which can be helpful in understanding what is going on. For example using:

std::cout << __PRETTY_FUNCTION__ << ": " << t <<std::endl ;

would results int following for variadic functions in the example (see it live):

void func(T, Args...) [T = int, Args = <double, char, std::basic_string<char>>]: 1
void func(T, Args...) [T = double, Args = <char, std::basic_string<char>>]: 2.5
void func(T, Args...) [T = char, Args = <std::basic_string<char>>]: a
void func(T) [T = std::basic_string<char>]: Hello

In Visual Studio you can use FUNCSIG.

Update Pre C++11

Pre C++11 the alternative for std::initializer_list would be std::vector or one of the other standard containers:

#include <iostream>
#include <string>
#include <vector>

template <class T>
void func1( std::vector<T> vec )
{
    for( typename std::vector<T>::iterator iter = vec.begin();  iter != vec.end(); ++iter )
    {
        std::cout << *iter << std::endl ;
    }
}

int main()
{
    int arr1[] = {10, 20, 30, 40} ;
    std::string arr2[] = { "hello", "world" } ; 
    std::vector<int> v1( arr1, arr1+4 ) ;
    std::vector<std::string> v2( arr2, arr2+2 ) ;

    func1( v1 ) ;
    func1( v2 ) ;
}

and the alternative for variadic templates would be variadic functions although they are not type-safe and in general error prone and can be unsafe to use but the only other potential alternative would be to use default arguments, although that has limited use. The example below is a modified version of the sample code in the linked reference:

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

void simple_printf(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);

    while (*fmt != '') {
        if (*fmt == 'd') {
            int i = va_arg(args, int);
            std::cout << i << '
';
        } else if (*fmt == 's') {
            char * s = va_arg(args, char*);
            std::cout << s << '
';
        }
        ++fmt;
    }

    va_end(args);
}


int main()
{
    std::string
        str1( "Hello" ),
        str2( "world" );

    simple_printf("dddd", 10, 20, 30, 40 );
    simple_printf("ss", str1.c_str(), str2.c_str() ); 

    return 0 ;
} 

Using variadic functions also comes with restrictions in the arguments you can pass which is detailed in the draft C++ standard in section 5.2.2 Function call paragraph 7:

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg (18.7). The lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are performed on the argument expression. After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer to member, or class type, the program is ill-formed. If the argument has a non-POD class type (clause 9), the behavior is undefined. [...]