how to move elements of an initializer_list?

Question

Let's say you have a variable of type std::vector<std::string> and you initialize it with an initializer list:

using V = std::vector<std::string>;
V v = { "Hello", "little", "world", "of", "move", "semantics" };

The compiler will create a temporary std::string for each string literal, create an initializer list over these and then call the ctor for V and create the vector. The ctor does not know that all those strings are temporaries, so it is copying each string.

I haven't found anything in the standard which allows the vector ctor to move the elements when they are temporaries.

Am I missing something or does using initializer lists lead to unnecessary copies? I am writing classes where this problem could lead to significantly inefficient code. Any technique to avoid unnecessary copies would be greatly appreciated.

Answer 1

There is no way to avoid the copying from an initializer_list<string>, because the standard defines the invocation of a constructor taking an initializer list argument, from a curly braces initializer as actual argument, as follows (emphasis added):

C++14 §8.5.4/5

” An object of type std::initializer_list<E> is constructed from an initializer list as if the implementation allocated a temporary array of N elements of type const E, where N is the number of elements in the initializer list

IMHO this is really unfortunate.

A workaround (for your own classes) is to accept initializer_list<char const*>.

---

Here's an example of the workaround applied to std::vector<string>. For that, where you don't control the class' code, it involves declaring a data array (actually an initializer_list) explicitly. This is just as with C++03, which the initializer list mechanism was intended to avoid:

#include <vector>
#include <initializer_list>
#include <iostream>
#include <iterator>              // std::begin, std::end
using namespace std;

struct My_string
{
    char const* const ps;

    My_string( char const* const s )
        : ps( s )
    {
        cout << "  My_string(*) <- '" << s << "'" << endl;
    }

    My_string( My_string const& other )
        : ps( other.ps )
    {
        cout << "  My_string(const&) <- '" << other.ps << "'" << endl;
    };

    My_string( My_string&& other )
        : ps( other.ps )
    {
        cout << "  My_string(&&) <- '" << other.ps << "'" << endl;
    };
};

auto main() -> int
{
    cout << "Making vector a." << endl;
    vector<My_string> const a   = {"a1", "a2", "a3"};
    cout << "Making data for vector b." << endl;
    auto const b_data           = { "b1", "b2", "b3" };
    cout << "Making vector b." << endl;
    vector<My_string> const b( begin( b_data ), end( b_data ) );
}

Output:

Making vector a.
  My_string(*) <- 'a1'
  My_string(*) <- 'a2'
  My_string(*) <- 'a3'
  My_string(const&) <- 'a1'
  My_string(const&) <- 'a2'
  My_string(const&) <- 'a3'
Making data for vector b.
Making vector b.
  My_string(*) <- 'b1'
  My_string(*) <- 'b2'
  My_string(*) <- 'b3'

Answer 2

After some thinking, I came up with a solution based on mutable. The other answer is still mostly correct, but one can create a proxy with a mutable member to get rid of the top-level const-ness and then move the elements from there. Methods taking an initializer list should therefore overload for a const-ref initializer list and an rvalue-ref version to know when they are allowed to move.

Here's a working example, it might look arbitrary at first but in my real-world use-case, it solved the problem.

#include <iostream>
#include <vector>

// to show which operations are called
struct my_string
{
    const char* s_;
    my_string( const char* s ) : s_( s ) { std::cout << "my_string(const char*) " << s_ << std::endl; }
    my_string( const my_string& m ) : s_( m.s_ ) { std::cout << "my_string(const my_string&) " << s_ << std::endl; }
    my_string( my_string&& m ) noexcept : s_( m.s_ ) { std::cout << "my_string(my_string&&) " << s_ << std::endl; }
    ~my_string() { std::cout << "~my_string() " << s_ << std::endl; }
};

// the proxy
struct my_string_proxy
{
    mutable my_string s_;

    // add all ctors needed to initialize my_string
    my_string_proxy( const char* s ) : s_( s ) {}
};

// functions/methods should be overloaded
// for the initializer list versions

void insert( std::vector<my_string>& v, const std::initializer_list<my_string_proxy>& il )
{
    for( auto& e : il ) {
        v.push_back( e.s_ );
    }
}

void insert( std::vector<my_string>& v, std::initializer_list<my_string_proxy>&& il )
{
    for( auto& e : il ) {
        v.push_back( std::move( e.s_ ) );
    }
}

int main()
{
    std::vector<my_string> words;
    insert( words, { {"Hello"}, {"initializer"}, {"with"}, {"move"}, {"support"} } );
}

Live example