CODEWITHSUNDEEP
stlc++11
towi
towi

Reputation: 22267

C++0x emplace_back with non-copyable Items

I wonder if I understood emplace_back correctly

#include <vector>
using namespace std;

struct Hero {
    Hero(const string&) {}
    Hero(const char*) {}
    Hero(int) {}
    // forbid a clone:
    Hero(const Hero&) = delete;
    Hero& operator=(const Hero&) = delete;
};

int main() {
    vector<Hero> heros1 = { "Bond", "Hulk", "Tarzan" }; // ERR: copies?

    vector<Hero> heros;
    heros.emplace_back( 5 );              // ERR: copies
    heros.emplace_back( string("Bond") ); // ERR: copies
    heros.emplace_back( "Hulk" );         // ERR: copies
}

Thus, I am really wondering If I understood emplace_back incorrectly: I though it would prevent to make a copy of Hero, because it creates the Item in-place.

Or is it a implementation error in my g++-4.7.0?

Upvotes: 10

Views: 3034

Answers (3)

SoapBox
SoapBox

Reputation: 20609

You need to define a move constructor and move-assignment operator, like this:

struct Hero {
    Hero(const string&) {}
    Hero(const char*) {}
    Hero(int) {}

    Hero(Hero&&) {}
    Hero& operator=(Hero&&) { return *this; }

    // forbid a clone:
    Hero(const Hero&) = delete;
    Hero& operator=(const Hero&) = delete;
};

This allows values of type Hero to be moved into the function. Move is usually faster than copy. If the type is neither copyable nor movable then you cannot use it in a std::vector.

Upvotes: 6

towi
towi

Reputation: 22267

Uh... I got it.

If I forbit to copy the Heros, I must allow them to move, if I want to put them in containers. How silly of me.

struct Hero {
    Hero(const string&) {}
    Hero(const char*) {}
    Hero(int) {}
    // no clone:
    Hero(const Hero&) = delete;
    Hero& operator=(const Hero&) = delete;
    // move only:
    Hero(Hero&&) {}
    Hero& operator=(Hero&&) {}
};

And all examples except for the initializer list work.

Upvotes: 3

Puppy
Puppy

Reputation: 146910

It's no implementation error- you did not provide a move constructor.

Upvotes: 4

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