CODEWITHSUNDEEP
c++c++11variadic-templates
djfm
djfm

Reputation: 2458

Use brace-enclosed initializer lists in a variadic template?

I'm trying to use brace-enclosed initializer lists in a variadic template function, but the compiler complains... am I asking too much or did I do something wrong?

This is best demonstrated by example:

struct Bracy
{
    Bracy(int i, int j)
    {
    }
};

struct Test
{

    void consumeOne(int i)
    {
    }

    void consumeOne(const Bracy & bracy)
    {
    }

    void consume()
    {
    }

    template<typename T, typename ...Values>
    void consume(const T & first, Values... rest)
    {
        consumeOne(first);
        consume(rest...);
    }

    template<typename ...Values>
    Test(Values... values)
    {
        consume(values...);
    }
};

void testVariadics()
{
    Test(7,{1,2}); //I'd like {1,2} to be passed to consumeOne(const Bracy & bracy)
}

GCC (4.7) says:

main.cpp:45:14: error: no matching function for call to ‘Test::Test(int, <brace-enclosed initializer list>)’

Upvotes: 7

Views: 2906

Answers (2)

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506905

A brace enclosed initializer list cannot be forwarded, so you are unfortunately out of luck.

Upvotes: 2

dans3itz
dans3itz

Reputation: 1615

This is a rough attempt at what you want...

#include <iostream>
#include <initializer_list>

struct Bracy {
    Bracy(int x, int y) {}
};

struct Test {
    void consumeOne(std::initializer_list<int>) { std::cout << "initializer list version (Bracy?)\n"; /* Bracy? */}

    void consumeOne(int) { std::cout << "int version\n"; }

    template<typename T>
    void consume(T t) { consumeOne(t); }

    template<typename T, typename ... Args>
    void consume(T first, Args ... args) {
        consumeOne(first);
        consume(args...);
    }

    template<typename ... Args>
    Test(Args ... args) {
        consume(args...);
    }
};

int
main(int argc, char** argv) {
    Test(1, std::initializer_list<int>{1,2}, 2, 3, std::initializer_list<int>{1,2});
    return 0;
}

output: int version
        initializer list version (Bracy?)
        int version
        int version
        initializer list version (Bracy?)

Upvotes: 0

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