C++23: Literal suffix for (signed) size_t

Let's continue our exploration of C++23 features! This week we discuss the extended language support for literal suffixes.

What is a literal suffix?

Literals can have an optional suffix which indicates the type of the literal. As such, one doesn't have to store the value in a variable of the desired type but can use the literal directly.

For example, if you need a long value and you don't want to rely on implicit conversions, you can pass 42L instead of passing 42.

While we can define our own user-defined literals, for integers, C++ provides quite a few literal suffixes:

• none means that the literal is an int
• U makes an integer unsigned
• L makes integers long
• LL males them long long
• ULL (or LLU) turns ints into unsigned long long ints

And C++23 is going to add one, or if combined with U then 2 elements to this list:

• Z turns an int into the sized version of std::size_t
• UZ turns an int into std::size_t

But why do we need this new Z literal suffix?

If you're an Almost Always Auto person, you probably shook your head quite often when you wanted to write a good old for loop. But even if you just had a look into legacy code's for loops, you probably saw too many messed up situations with loop indexes.

Let's have a look at a simple situation:

#include <vector>

int main() {
  std::vector<int> v{0, 1, 2, 3};
    for (auto i = 0; i < v.size(); ++i) {
      /* use both i and v[i] */
    }
}

We try to use auto for the loop index, but we got a compiler warning! std::vector<T>::size() returns a std::vector<T>::size_type, usually std::size_t that is an unsigned type. At the same time, 0 is deduced as a signed integer. Comparing a signed with an unsigned type leads to a compiler warning. Hopefully, you don't tolerate compiler warnings in your project, so we consider that the above example does not compile.

In case, you want to store the size of the vector for optimization reasons, you even get a hard error, reminding you that the auto education for i and s was not consistent!

#include <vector>

int main() {
  std::vector<int> v{0, 1, 2, 3};
    for (auto i = 0, s = v.size(); i < s; ++i) {
      /* use both i and v[i] */
    }
}

What if 0u is used for initializing i? It depends on whether you have a helper variable to store the size and on your system.

The worst case is that i will be truncated on a 64-bit system as 0u is deduced as an unsinged int, while s is a long unsigned int. In a better situation, you get a compilation error because of this:

#include <vector>

int main() {
  std::vector<int> v{0, 1, 2, 3};
    for (auto i = 0u, s = v.size(); i < s; ++i) {
      /* use both i and v[i] */
    }
}
/*
main.cpp: In function 'int main()':
main.cpp:5:10: error: inconsistent deduction for 'auto': 'unsigned int' and then 'long unsigned int'
    5 |     for (auto i = 0u, s = v.size(); i < s; ++i) {
      |   
*/

These were the simple examples borrowed from the accepted proposal, but you can find there many more. In general, with an existing set of literal suffixes, you can run into situations when you want the compiler to deduce the type for you for an integer literal because

• comparing signed with unsigned elements is unsafe
• and you cannot replace std::size_t with ul (unsigned long) because you can run into narrowing/truncating situations when switching between 32-bit and 64-bit systems

To avoid the problems, you either have to use some verbose casts (mostly static_cast) or introduce a helper variable without relying on auto type deduction.

As mentioned in the beginning, [P0330R8] finally solves this problem by introducing Z and UZ. Z introduces the signed version of std::size_t and UZ the unsigned version.

With that, our previous examples should compile without any problem and unpleasant surprises as such:

#include <vector>

int main() {
  std::vector<int> v{0, 1, 2, 3};
    for (auto i = 0UZ, s = v.size(); i < s; ++i) {
      /* use both i and v[i] */
    }
}

Just make sure that you compile with the option -std=c++2b.

Conclusion

In this article, we saw why it's difficult to use literal suffixes and auto type deduction for good old loops and how P0330R8 solves this situation by introducing Z/UZ in C++23 to denote std::size_t.

Where do you think the signed version of size_t comes in handy?

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