C++ unions with string types

Question

Reading cppreference.com's page on Unions, an example is presented as below:

union S
{
    std::string str;
    std::vector<int> vec;
    ~S() {} // needs to know which member is active, only possible in union-like class 
};          // the whole union occupies max(sizeof(string), sizeof(vector<int>))

The motivation for boost::variant presents that it is illegal to have a std::string in a union and so does the answer to an SO question why-compiler-doesnt-allow-stdstring-inside-union.

Which one is right? and why does the code in the cpp-reference work?

Lightness Races in Orbit · Accepted Answer · 2019-05-17T12:24:45.093

2

You should read all the answers on the [old] Q&A you linked, not just one.

kennytm's answer explains that the rule was relaxed in C++11, and gives an example.

edited May 17 '19 at 12:24

answered May 17 '19 at 11:12

Lightness Races in Orbit

378,754
76
643
1,055

Thanks, I was trying to understand the motiviation behind `boost::variant` really. – Samer Tufail May 17 '19 at 11:15
@SamerTufail Bear in mind that _Boost.Variant_ predates C++11 by a lot. – Lightness Races in Orbit May 17 '19 at 11:23
2

The answer doesn't say that boost variant works like this. It says that boost variant is an existing implementation of tagged union. It is actually not implemented using a `union` at all. – eerorika May 17 '19 at 11:29
@eerorika Granted (and fixed) Thanks! – Lightness Races in Orbit May 17 '19 at 12:24

C++ unions with string types

1 Answers1