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C++17 in Detail: A Deep Dive

Exception safety guarantees

Explore the concept of exception safety guarantees within std::variant in C++17. Understand how exceptions can affect the variant's state during assignment and emplacement, what a valueless-by-exception variant means, and the implications for accessing variant values after exceptions. This lesson helps you safely manage and use std::variant in your C++ programs.

We'll cover the following...

So far everything looks nice and smooth… but what happens when there’s an exception during the creation of the alternative in a variant?

Example:

C++ 17
#include <iostream>
#include <string>
#include <variant>
using namespace std;
class ThrowingClass
{
public:
    explicit ThrowingClass(int i) {
        if (i == 0) throw int (10);
    }
    operator int () {
        throw int(10);
    }
};
int main(int argc, char** argv)
{
    std::variant<int, ThrowingClass> v;
// change the value:
    try
    {
        v = ThrowingClass(0);
    }
    catch (...)
    {
        std::cout << "catch(...)\n";
// we keep the old state!
        std::cout << v.valueless_by_exception() << '\n';
        std::cout << std::get<int>(v) << '\n';
    }
// inside emplace
    try
    {
        v.emplace<0>(ThrowingClass(10)); // calls the operator int
    }
    catch (...)
    {
        std::cout << "catch(...)\n";
// the old state was destroyed, so we're not in invalid state!
        std::cout << v.valueless_by_exception() << '\n';
    }
    return 0;
}

In the first case - with the assignment operator - the ...