C++17 emplace demo

emplace.cpp

// démonstration emplace c++11 vs. c++17
// rene-d 2019

//#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <unordered_map>
using namespace std;

struct toto
{
    int a;

    explicit toto(int _a)
    {
        cout << "toto() int " << _a << endl;
        a = _a;
    }

    toto()
    {
        cout << "toto()" << endl;
        a = 1;
    }

    virtual ~toto()
    {
        cout << "~toto()" << endl;
    }
};

void test_vector()
{
    vector<toto> t;

#if _LIBCPP_STD_VER > 14          // C++17 et suivants (constante clang++)
    auto &&r1 = t.emplace_back(); // auto& fonctionne aussi
    r1.a = 100;
#else
    auto &&r2 = t.emplace(t.end());
    r2->a = 1000;
#endif

    for (auto &&it : t)
        cout << "vector " << it.a << endl;
}

void test_map()
{
    unordered_map<int, toto> m;

    auto &&r2 = m.emplace(std::piecewise_construct,
                          std::forward_as_tuple(20),
                          std::forward_as_tuple());
    r2.first->second.a = 200;

#if _LIBCPP_STD_VER > 14
    // c++17 autorise une syntaxe plus simple
    auto &&r3 = m.try_emplace(30);
    r3.first->second.a = 300;
#endif

    for (auto &&it : m)
        cout << "map " << it.first << " " << it.second.a << endl;
}

int main()
{
    test_vector();
    cout << "---------------------------------" << endl;
    test_map();
}

/* sortie C++11
toto()
vector 1000
~toto()
---------------------------------
toto()
map 20 200
~toto()
*/

/* sortie C++17
toto()
vector 100
~toto()
---------------------------------
toto()
toto()
map 30 300
map 20 200
~toto()
~toto()
*/