Ivan Rubinson Ivan Rubinson - 29 days ago 11
C++ Question

Compilation error when trying to find_if an unique_ptr in a vector

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)



  1. Which compiler is bugged?

  2. How do I erase a pointer from a
    std::vector
    of
    std::unique_ptr
    correctly?


Answer

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