Compilation error when trying to find_if an unique_ptr in a vector

Question

This code:

#include <memory>
#include <vector>
#include <algorithm>

struct Foo
{
    int bar;

    Foo(const int val) :
        bar(val)
    {
    }
};

int main() {
    std::vector<std::unique_ptr<Foo>> vec;
    vec.emplace_back(std::make_unique<Foo>(42));
    Foo* ptr = vec.back().get();
    auto& it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr<Foo>& p)
    {
        return p.get() == ptr;
    });
    if (it != vec.end())
    {
        vec.erase(it);
    }

    return 0;
}

Works fine in MSVC, but errors out in GCC 5.1:

prog.cpp: In function 'int main()':

prog.cpp:19:25: error: invalid initialization of non-const reference of type '__gnu_cxx::__normal_iterator*, std::vector > >&' from an rvalue of type '__gnu_cxx::__normal_iterator*, std::vector > >' auto& it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr& p)

Which compiler is bugged?
How do I erase a pointer from a std::vector of std::unique_ptr correctly?

"bugged" implies it's unintentional behavior. It's not. The MSVC devs are well aware of this and have no intention of ever changing it. — Benjamin Lindley
– Benjamin Lindley, Commented Nov 7, 2016 at 0:17
However, if you want to enforce strict standards compliance (to the best of the compiler's current ability), use the /Za flag. — Benjamin Lindley
– Benjamin Lindley, Commented Nov 7, 2016 at 0:25

krzaq · Accepted Answer · 2016-11-07 00:01:42Z

10

gcc is correct here. You can't initialize a lvalue reference with an rvalue, and you're doing it for the it reference to an iterator (std::find_if returns an rvalue)

auto& it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr<Foo>& p)
    ^

Either make it an object:

auto it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr<Foo>& p)

demo

or a const reference:

auto const& it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr<Foo>& p)

demo

Other than that, your code for erasing an element from a vector is correct

edited Nov 7, 2016 at 0:01

answered Nov 6, 2016 at 23:54

krzaq

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Comments

wally · Accepted Answer · 2016-11-07 00:29:08Z

VS2015 will emit a warning that a nonstandard extension is being used if you turn on level 4 warnings /W4:

warning C4239: nonstandard extension used:
note: A non-const reference may only be bound to an lvalue

#pragma warning( push, 3 )  
#include <memory>
#include <vector>
#include <algorithm>
#pragma warning( pop ) 
#pragma warning( disable : 4710 )

struct Foo
{
    int bar;

    Foo(const int val) :
        bar(val)
    {
    }
};

int main() {
    std::vector<std::unique_ptr<Foo>> vec;
    vec.emplace_back(std::make_unique<Foo>(42));
    Foo* ptr = vec.back().get();
    auto& it = std::find_if(vec.begin(), vec.end(), [&](std::unique_ptr<Foo>& p)
    {
        return p.get() == ptr;
    });
    if(it != vec.end())
    {
        vec.erase(it);
    }

    return 0;
}

Produces:

1>c:\users\flatmouse\documents\visual studio 2015\projects\project79\project79\source.cpp(25): warning C4239: nonstandard extension used: 'initializing': conversion from 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<std::unique_ptr<Foo,std::default_delete<_Ty>>>>>' to 'std::_Vector_iterator<std::_Vector_val<std::_Simple_types<std::unique_ptr<Foo,std::default_delete<_Ty>>>>> &'
1>          with
1>          [
1>              _Ty=Foo
1>          ]
1>  c:\users\flatmouse\documents\visual studio 2015\projects\project79\project79\source.cpp(25): note: A non-const reference may only be bound to an lvalue

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