C++ Basics: Probe a Variant Without Throwing

Use std::get_if when one variant alternative is interesting and other alternatives should follow an ordinary fallback path.

Prepare a scratch directory

Tested on Linux x86_64 with GCC 14.2.0. Install a C++20-capable compiler and standard-library development headers, with g++ on PATH. You also need Bash, a text editor, and the mktemp, rm, and rmdir utilities. No third-party library is required.

Keep the same terminal open. Stop if setup fails, before saving the source:

start_directory="$PWD"
work_directory="$(mktemp -d)" || exit 1
cd -- "$work_directory" || exit 1

Run the example

Save this as variant-get-if-probe-demo.cpp:

#include <iostream>
#include <string>
#include <variant>

using Setting = std::variant<int, std::string>;

void show(const Setting& setting)
{
    if (const int* value = std::get_if<int>(&setting)) {
        std::cout << "numeric: " << *value << '\n';
    } else {
        std::cout << "not numeric\n";
    }
}

int main()
{
    Setting setting{0};
    show(setting);
    setting = std::string{"automatic"};
    show(setting);
    const Setting* absent = nullptr;
    std::cout << std::boolalpha
              << "null input: " << (std::get_if<int>(absent) == nullptr) << '\n';
}

Compile and run it. The program runs only after compilation succeeds:

g++ -std=c++20 -Wall -Wextra -Werror -pedantic variant-get-if-probe-demo.cpp -o variant-get-if-probe-demo &&
./variant-get-if-probe-demo

Observed output:

numeric: 0
not numeric
null input: true

Read the result

Zero is a successfully found integer, so the first call prints it. The string alternative follows the fallback. A null variant pointer also produces a null result.

Apply it carefully

The type-based overload requires the requested type to occur exactly once in the variant. Test the returned pointer before dereferencing it. Do not retain it across destruction or a change of active alternative. Unlike get, get_if reports a mismatch with nullptr rather than bad_variant_access.

The C++ working draft describes the relevant rules. This example builds in C++20 mode; the current working draft also documents later additions.

Clean up

Remove only the source and executable created here, then return to the starting directory:

rm -- variant-get-if-probe-demo.cpp variant-get-if-probe-demo
cd -- "$start_directory" || exit 1
rmdir -- "$work_directory"
unset start_directory work_directory

If the directory is not empty, inspect its remaining contents before removing anything else.

Acknowledgements

This post was created with assistance from AI.