optional<const string &>::value_or returns reference to temp

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
cpp
Domain
backend-api-design

Research direction

Start in boost/optional.hpp and reproduce the issue with the supplied program using Boost 1.69 and GCC 8.2. Trace value_or for boost::optional<const std::string&>, then verify that the shown call no longer returns a reference to a temporary or crashes when the optional is populated.

Written by the indexing model from the issue text.

Description

I'm using boost::optional<const string &> and found some pitfalls when using value_or(char *).

Basically since value_or takes the "or" party as a template type the conversion from char * to string happens inside the function call and then a reference to that temporary is returned, leading to UB and in my case a crash.

Furthermore, even if the optional is populated and the alternative isn't use, it seems that some sort of copy of the value is still required because of the use of the ?: and then a similar crash occurs.

I think a reasonable workaround would be to have the value_or for references take the alternative value (r) only as const T & instead of as a type param. This would require the implicit conversion to happen at the call site and the function would not create a temporary.

This doesn't avoid all possible dangling references but it improves things.

Here is a short reproduction. I'm using boost 1.69, gcc 8.2:

#include <iostream>
#include <cstdlib>
#include <boost/optional.hpp>

int main()
{
    const std::string input = "a good string";
    boost::optional<const std::string &> opt = input;

    std::string output = opt.value_or("alternative");  // segfault here

    std::cout << output << std::endl;
}

Note I do get a warning about returning a reference to a temporary. But usually we disable warnings within boost because there's so much noise.

Dominant language
C++
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