Cannot initialize const int from unpacked tuple

Question

Question is really simple, why is this code not working:

#include <tuple>

int main( int argc, char* argv[]) {
    const int a,b = std::tie(std::make_pair(1,2));
    return EXIT_SUCCESS;
}

g++ gives me this error:

./test.cpp: In function ‘int main(int, char**)’: ./test.cpp:4:13: error: uninitialized const ‘a’ [-fpermissive] const int a,b = std::tie(std::make_pair(1,2)); ^ ./test.cpp:4:42:

error: cannot bind non-const lvalue reference of type ‘std::pair&’ to an rvalue of type ‘std::pair’
const int a,b = std::tie(std::make_pair(1,2));

I cannot get any tuple-like return by value, using this kind of pattern (either const or non const). Is it a better way to do what I am trying to achieve here ?

Answer 1

const int a,b = std::tie(...)

This isn't doing what you think it is. It's creating two const int variables:

• a, uninitialized

• b, initialized to std::tie(...)

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The proper way of using std::tie is as follows:

int a, b;
std::tie(a, b) = std::make_pair(1, 2);

Note that you need a and b to be already declared and non-const.

---

In C++17, you can use structured bindings instead:

const auto [a, b] = std::make_pair(1, 2);