Some solutions from "Declaring an object before initializing it in c++" https://stackoverflow.com/questions/800368/declaring-an-object-before-initializing-it-in-c/62247236#62247236

animals.cpp

/* Solutions to https://stackoverflow.com/questions/800368/declaring-an-object-before-initializing-it-in-c/62247236#62247236
 *
 * Uses c++17 for some solutions
 *
 * Output:
 *
 *     $ g++ --std=c++17 -o animals animals.cpp && ./animals
 *     Naive Method
 *     ------------
 *     Default Constructing Animal at 0x7ffee3fc20d0
 *     Constructing puppies at 0x7ffee3fc20f0
 *     Deconstructing puppies at 0x7ffee3fc20f0
 *     Deconstructing puppies at 0x7ffee3fc20d0
 *
 *     Opt Method
 *     ----------
 *     Constructing toads at 0x7ffee3fc2160
 *     optional size: 40
 *     original size: 32
 *     Deconstructing toads at 0x7ffee3fc2160
 *
 *     Direct Method
 *     ----------
 *     Constructing Butterfly at 0x7ffee3fc2140
 *     Deconstructing Butterfly at 0x7ffee3fc2140
 */

#include <string>
#include <iostream>
#include <memory>
#include <optional>

class Animal {
public:
    std::string name;
    Animal(): name("Animal") {
        std::cout << "Default Constructing " << name << " at " << this << std::endl;
    }
    Animal(Animal const& a): name(a.name) {
        std::cout << "Copy Constructing at " << this << " from " << &a << std::endl;
    }
    Animal(std::string name): name(name) {
        std::cout << "Constructing " << name << " at " << this << std::endl;
    }
    ~Animal() {
        std::cout << "Deconstructing " << name << " at " << this << std::endl;
    }
};

bool _happy = false;
bool happyDay() {
    _happy = !_happy;
    return _happy;
}

void naive() {
    Animal a;
    if( happyDay() )
        a = Animal(std::string("puppies" )); //constructor call
    else
        a = Animal( "toads" );
}

void opt1() {
    std::optional<Animal> a;
    if( happyDay() )
        a = Animal("puppies" ); //constructor call
    else
        a = Animal( "toads" );
}

void opt() {
    std::optional<Animal> a;
    if( happyDay() )
        a.emplace("puppies" ); //constructor call
    else
        a.emplace( "toads" );

    std::cout << "optional size: " << sizeof(a) << std::endl;
    std::cout << "original size: " << sizeof(a.value()) << std::endl;
}

void direct() {
    Animal a("Butterfly");
}

int main(int argc, const char* argv[]) {
    std::cout << "Naive Method" << std::endl;
    std::cout << "------------" << std::endl;
    naive();
    std::cout << std::endl;

    std::cout << "Opt Method" << std::endl;
    std::cout << "----------" << std::endl;
    opt();
    std::cout << std::endl;

    std::cout << "Direct Method" << std::endl;
    std::cout << "----------" << std::endl;
    direct();
    std::cout << std::endl;

    return 0;
}