std::pair returned by std::transform resulting in segfault

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I'm trying to transform a vector of strings to a vector of pairs of strings, and I was getting segfault. I've tried to narrow it down to a simple test case (below), and I'm sure it's likely to do with the memory allocation:

#include <string>
#include <vector>
#include <utility>

std::pair<std::string, std::string>
newP( const std::string& foo) {
  return std::make_pair(std::string("Foo"), std::string("Bar"));
}

int main()
{
  std::vector<std::string> vec;
  std::vector<std::pair<std::string, std::string>> pairs;

  vec.push_back("Hello, ");
  vec.push_back("World!");

  std::transform(vec.cbegin(), vec.cend(), pairs.begin(), newP);
}

I get following segfault on FreeBSD 13.1:

Program received signal SIGSEGV, Segmentation fault.
Address not mapped to object.
0x0000000000205fd5 in std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long (this=0x0) at /usr/include/c++/v1/string:1456
1456            {return bool(__r_.first().__s.__size_ & __short_mask);}
(gdb) bt
#0  0x0000000000205fd5 in std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__is_long (this=0x0) at /usr/include/c++/v1/string:1456
#1  0x0000000000205f0d in std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__move_assign (this=0x0, __str="Foo") at /usr/include/c++/v1/string:2463
#2  0x0000000000205ee1 in std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::operator= (this=0x0, __str="Foo") at /usr/include/c++/v1/string:2485
#3  0x0000000000205ded in std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >::operator= (
    this=0x0, __p=...) at /usr/include/c++/v1/__utility/pair.h:277
#4  0x0000000000203fe5 in std::__1::transform<std::__1::__wrap_iter<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const*>, std::__1::__wrap_iter<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >*>, std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > (*)(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)> (__first=..., __last=...,
    __result=..., __op=0x203c20 <newP(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)>) at /usr/include/c++/v1/__algorithm/transform.h:29
#5  0x0000000000203d96 in main () at pair.cc:19

Could someone please explain what I'm doing wrong ? And if possible, what would be the best way to fix this. I'm fairly out of touch with modern C++.

Thanks!

memory c++11 segmentation-fault stl c++
1 Answers

The vector pairs is empty, and pairs.begin() will return the pairs.end() iterator which can't be dereferenced.

Set the size of pairs to the same size as vec before calling transform:

pairs.resize(vec.size());
std::transform(vec.cbegin(), vec.cend(), pairs.begin(), newP);

An alternative that has been mentioned is to preallocate memory and then use std::back_inserter to append to the vector:

pairs.reserve(vec.size());
std::transform(vec.cbegin(), vec.cend(), std::back_inserter(pairs), newP);

While the reserve call isn't strictly needed, it will avoid extra re-allocations and copying that might otherwise happen when adding new elements to the vector.

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