For some reason, I get different result when comparing the return value from vector::erase inside a if statement or if I store the value first and then compare. But it seems to only happen with g++.
This is built on Ubuntu 20.04 AMD64 using g++, libstdc++ and with -std=c++14 flag.
g++ -std=c++14 foo.cpp && ./a.out
This will return false
#include <vector>
#include <iostream>
#include <algorithm>
int main()
{
std::vector<int> v{0, 1, 8, 3, 8, 5, 8, 7,8, 9};
int thing_id{9};
std::vector<int>::iterator const cit{
std::remove_if(v.begin(),
v.end(),
[thing_id](int const& thing) -> bool {
return thing == thing_id;
})};
if (v.erase(cit, v.cend()) == v.cend()) {
std::cout << "true\n";
return true;
}
else {
std::cout << "false\n";
return false;
}
}
This will return true
#include <vector>
#include <iostream>
#include <algorithm>
int main()
{
std::vector<int> v{0, 1, 8, 3, 8, 5, 8, 7,8, 9};
int thing_id{9};
std::vector<int>::iterator const cit{
std::remove_if(v.begin(),
v.end(),
[thing_id](int const& thing) -> bool {
return thing == thing_id;
})};
auto const prev_end = v.erase(cit, v.cend());
if (prev_end == v.cend()) {
std::cout << "true\n";
return true;
}
else {
std::cout << "false\n";
return false;
}
}
When building for QNX ARM64 with QCC and libc++, both snippets will return true.
Why is this? Should not the comparisons be deterministic either or? Can someone please explain to me what is going on here?
vectorelements are simpleints and not objects, You can simplify your use ofstd::remove_if()by usingstd::remove()instead, you don't need the lambda at all, eg:std::remove(v.begin(), v.end(), thing_id);