casting unsigned to complex causes sign-conversion warning

Question

Consider the following code:

#include <complex>

int main()
{
    unsigned u = 1u;
    auto result = static_cast<std::complex<int>>(u);
    return 0;
}

Compiling with

g++ -std=c++11 -Werror -Wsign-conversion -o a.out source_file.cpp

Causes compile error

source_file.cpp: In function ‘int main()’:
source_file.cpp:8:51: error: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Werror=sign-conversion]
     auto result = static_cast<std::complex<int>>(u);
                                                   ^

clang reports a similar error

source_file.cpp:6:50: error: implicit conversion changes signedness: 'unsigned int' to 'const value_type' (aka 'const int') [-Werror,-Wsign-conversion]
    auto result = static_cast<std::complex<int>>(u);
                  ~~~~~~~~~~~                    ^

The error does not make much sense at first sight, what am I missing?

Compiles successfully for me. Check [here](https://godbolt.org/z/bRP59_) — NutCracker, Jan 24 '20 at 10:27
_The error does not make much sense_ — Why do you think so? With `-Werror`, warning are treated as errors, which is exactly what you are getting. — Daniel Langr, Jan 24 '20 at 10:28
@DanielLangr I thought it makes no sense because it doesn't happen when casting unsigned to int (instead of complex) — misev, Jan 24 '20 at 10:30
@NutCracker You omitted the OP's flags `-Werror -Wsign-conversion`. — Daniel Langr, Jan 24 '20 at 10:30
@misev The warning is there alright, its for the same reason that `int i = u` gives the same warning — darune, Jan 24 '20 at 10:32
@darune but `int i = static_cast(u)` works fine, so I'm mainly confused by the unclear discrepancy in behavior. — misev, Jan 24 '20 at 10:35
@misev The constructor of `std::complex` has a first parameter of type `const int&`, which is initialized with `u` argument. This is where the problem occurs. If you write `static_cast>(static_cast(u));`, then, then the [problem disappears](https://godbolt.org/z/YHywvF). — Daniel Langr, Jan 24 '20 at 10:36
@misev that due to the cast - ie. you telling c++ that it's ok - you should not get a conversion warning *at the cast level* — darune, Jan 24 '20 at 10:37

score 2 · Accepted Answer · answered Jan 24 '20 at 10:42

2

You get a conversion warning not from the cast, but from inside construction of std::complex.

To 'fix' your example you should instead do:

auto result = std::complex<int>{static_cast<int>(u)};

answered Jan 24 '20 at 10:42

darune

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casting unsigned to complex causes sign-conversion warning

1 Answers1