How to Deal with "Signed/Unsigned Mismatch" Warnings (C4018)

C++ warning C4018: '' : signed/unsigned mismatch

Replace all the definitions of int i with size_t i.

std::vector<T>::size() returns the type size_t which is unsigned (since it doesn't make sense for containers to contain a negative number of elements).

warning C4018: '' : signed/unsigned mismatch

length() probably returns size_t or unsigned long and you are comparing it with signed long.
Change

long lDelFront = 0, lDelBack = 0;

to

size_t lDelFront = 0;
size_t lDelBack = 0;

to avoid signed/unsigned comparison

“signed/unsigned mismatch” warnings (C4018) with decrementing for loop

Firstly, learn a few well-established idioms that'd allow you to write decrementing cycles using unsigned types. This is an absolutely essential skill in C and C++. E.g.

for ([some unsigned type] i = N; i-- > 0; )
{
  // Process i-th element
}

or

for ([some unsigned type] i = N; i > 0; )
{
  --i;
  // Process i-th element
}

(Note, that the initial value of i is N, not N - 1.)

Or you can even use a "more natural" approach

for ([some unsigned type] i = N - 1; i != -1; --i)
{
  // Process i-th element
}

Secondly, start using the proper unsigned type for vector indexing. In your case

for (auto i = vector.size() - 1; i != -1; --i)

or

for ([your vector type]::size_type i = vector.size() - 1; i != -1; --i)

That will solve your warning issue.

Alternatively, you can choose a "loser's way out" and just suppress the warning by using an explicit cast

if ((unsigned) i < vector.size() / 2) 

or just do #pragma warning(disable: 4018).

How to deal with “signed/unsigned mismatch” warnings (C4018, no loop)?

To get answer which value is bigger use signed arbitrary arithmetic types like boost::cpp_int

auto v1 = int(-3);
auto v2 = size_t(31);
boost::cpp_int(v1) >= boost::cpp_int(v2);

If you have access to 80bit MMX registers then consider using them to store both values as signed and compare.

replacing macro with a function causes signed/unsigned mismatch warning

Move the problem out of your code by calling std::cmp_not_equal

const auto f = [](auto&& lhs, auto&& rhs) {
     if (std::cmp_not_equal(lhs, rhs)) { blah blah }
};

This family of functions, added in C++20, is defined in a way that properly compares integer arguments even in the presence of a signed/unsigned mismatch.

Now the Standard library author is responsible for writing "all that template gunk" and all you need to do extra is #include <utility>

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For the specific case of assertions, you need a macro anyway to capture information.

#define ASSERT_EQ(actual, expected) do { \
      if (lhs == rhs) break; \
      log_assertion_failure(__FILE__, __LINE__, #actual, actual, expected); \
  } while(0)

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