Probably a bug in C++

Code1

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

int main(){
    function <bool()> func=[](){
      vector <bool> a(1,true);
      return a[0];
    };
    
    for(int i=0;i<10;i++)
        cout<<func()<<'\n';

    return 0;
}

Look at this code. It seems, the expected output is always 1. However, in

Codeforces
- G++20
- G++17
- G++14
- VC++2017
in my local Windows G++ 8.1.0
in my local Debian G++ 10.2.1

the output is always 0. In Codeforces::Clang++17 & Clang++20 the output is

Clang++

Code2

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

int main(){
    function <bool()> func=[](){
      vector <bool> a(1,true);
      bool t=a[0];
      return t;
    };
    
    for(int i=0;i<10;i++)
        cout<<func()<<'\n';

    return 0;
}

Now look at this code. Here in all previously mensioned compilers (including Clang++) the output is always 1.

How found

Комментарии (6)

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ivan100sic

21 месяц назад, # |

+184

You are actually invoking undefined behavior.

The lambda on the RHS actually doesn't return bool but std::vector<bool>::reference. When the function returns, the vector goes out of scope and the attempt to read the value is UB. If you change the lambda return signature to bool or cast (bool)a[0] it'll work.

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CodingKnight

← Rev. 5 →

The following update works fine on all Codeforces Custom Test C++ compilers.

Update

#include <iostream>
#include <vector>
using namespace std;

int main() {
    const auto func = []() {
        vector<bool> a(1,true);
        return int(a[0]); };
    for (int i = 0; i < 10; ++i)
        cout<< func() << endl;
    return 0; }

I often use the const auto <lambda_expression_name> = ... syntax to let the compiler infer the return-value type automatically. This works fine, except in some cases when the inferred type should be a reference and when the lambda expression is defined recursively.