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 ?

std::tie does not "unpack" tuples and C++ is not Python. This looks like you are writing C++ based on guessing, which does not work. — Baum mit Augen
– Baum mit Augen ♦, Commented Oct 25, 2017 at 12:03

Vittorio Romeo · Accepted Answer · 2017-10-25 12:04:06Z

11

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(...)

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);

answered Oct 25, 2017 at 12:04

Vittorio Romeo

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3 Comments

Tobbey Over a year ago

Yes, I am really dumb, I just realized that 1 minute ago. Cannot delete question now.

StoryTeller - Unslander Monica Over a year ago

@Tobbey - Good. Vittorio doesn't deserve to have his effort go down the drain because you had an "oops" moment. That's why SO prohibits you from deleting.

Tobbey Over a year ago

You are right, anyway despite my comment Vittorio Romeo answer is interesting because I discovered that c++17 allows to do exactly what I want.

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