How do I deal with "signed/unsigned mismatch" warnings (C4018)?

Question

It's all in your things.size() type. It isn't int, but size_t (it exists in C++, not in C) which equals to some "usual" unsigned type, i.e. unsigned int for x86_32.

Operator "less" (<) cannot be applied to two operands of different sign. There's just no such opcodes, and standard doesn't specify, whether compiler can make implicit sign conversion. So it just treats signed number as unsigned and emits that warning.

It would be correct to write it like

for (size_t i = 0; i < things.size(); ++i) { /**/ }

or even faster

for (size_t i = 0, ilen = things.size(); i < ilen; ++i) { /**/ }

Ideally, I would use a construct like this instead:

for (std::vector<your_type>::const_iterator i = things.begin(); i != things.end(); ++i)
{
  // if you ever need the distance, you may call std::distance
  // it won't cause any overhead because the compiler will likely optimize the call
  size_t distance = std::distance(things.begin(), i);
}

This a has the neat advantage that your code suddenly becomes container agnostic.

And regarding your problem, if some library you use requires you to use int where an unsigned int would better fit, their API is messy. Anyway, if you are sure that those int are always positive, you may just do:

int int_distance = static_cast<int>(distance);

Which will specify clearly your intent to the compiler: it won't bug you with warnings anymore.

If you can't/won't use iterators and if you can't/won't use std::size_t for the loop index, make a .size() to int conversion function that documents the assumption and does the conversion explicitly to silence the compiler warning.

#include <cassert>
#include <cstddef>
#include <limits>

// When using int loop indexes, use size_as_int(container) instead of
// container.size() in order to document the inherent assumption that the size
// of the container can be represented by an int.
template <typename ContainerType>
/* constexpr */ int size_as_int(const ContainerType &c) {
    const auto size = c.size();  // if no auto, use `typename ContainerType::size_type`
    assert(size <= static_cast<std::size_t>(std::numeric_limits<int>::max()));
    return static_cast<int>(size);
}

Then you write your loops like this:

for (int i = 0; i < size_as_int(things); ++i) { ... }

The instantiation of this function template will almost certainly be inlined. In debug builds, the assumption will be checked. In release builds, it won't be and the code will be as fast as if you called size() directly. Neither version will produce a compiler warning, and it's only a slight modification to the idiomatic loop.

If you want to catch assumption failures in the release version as well, you can replace the assertion with an if statement that throws something like std::out_of_range("container size exceeds range of int").

Note that this solves both the signed/unsigned comparison as well as the potential sizeof(int) != sizeof(Container::size_type) problem. You can leave all your warnings enabled and use them to catch real bugs in other parts of your code.

You can use:

1. size_t type, to remove warning messages
2. iterators + distance (like are first hint)
3. only iterators
4. function object

For example:

// simple class who output his value
class ConsoleOutput
{
public:
  ConsoleOutput(int value):m_value(value) { }
  int Value() const { return m_value; }
private:
  int m_value;
};

// functional object
class Predicat
{
public:
  void operator()(ConsoleOutput const& item)
  {
    std::cout << item.Value() << std::endl;
  }
};

void main()
{
  // fill list
  std::vector<ConsoleOutput> list;
  list.push_back(ConsoleOutput(1));
  list.push_back(ConsoleOutput(8));

  // 1) using size_t
  for (size_t i = 0; i < list.size(); ++i)
  {
    std::cout << list.at(i).Value() << std::endl;
  }

  // 2) iterators + distance, for std::distance only non const iterators
  std::vector<ConsoleOutput>::iterator itDistance = list.begin(), endDistance = list.end();
  for ( ; itDistance != endDistance; ++itDistance)
  {
    // int or size_t
    int const position = static_cast<int>(std::distance(list.begin(), itDistance));
    std::cout << list.at(position).Value() << std::endl;
  }

  // 3) iterators
  std::vector<ConsoleOutput>::const_iterator it = list.begin(), end = list.end();
  for ( ; it != end; ++it)
  {
    std::cout << (*it).Value() << std::endl;
  }
  // 4) functional objects
  std::for_each(list.begin(), list.end(), Predicat());
}