Checking if object copying is taking place each time std::tie is called. The answer: TestObject is not copied. Good news, say, if we want to use std::tie for maps (with complex keys...)

std::tie copy test

#include <string>
#include <tuple>
#include <iostream>

struct TestObject
{
    std::string s;
    int i;
    
    TestObject(const std::string &s, int i)
    :
    s(s),
    i(i)
    {
        std::cout << "TestObject CREATED: " << s << std::endl;
    }
    
    ~TestObject()
    {
        std::cout << "TestObject DESTROYED: " << s << std::endl;
    }
    
    bool operator<(const TestObject &rhs) const
    {
        return std::tie(s, i) < std::tie(rhs.s, rhs.i);
    }
};

struct TestKey
{
    TestObject o;
    float f;
    
    TestKey(const TestObject &o, float f)
    :
    o(o),
    f(f)
    {}
    
    bool operator<(const TestKey &rhs) const
    {
        return std::tie(o, f) < std::tie(rhs.o, f);
    }
};


class Test
{
public:
    static void run()
    {
        TestKey k1(TestObject("foo", 123), 0.5f);
        TestKey k2(TestObject("bar", 123), 0.5f);
        
        if ((k1 < k2) || (k2 < k1))
        {
            std::cout << "!=" << std::endl;
        }
        else
        {
            std::cout << "=" << std::endl;
        }
    }
};