Design and evolution of constexpr in C++

constexpr is one of the magic keywords in modern C++. You can use it to create code, that is then executed before the compilation process ends. This is the absolute upper limit for software performance.

Design and evolution of constexpr in C++

by Evgeny Shulgin

From the article:

The authors suffered greatly from the inability to use STL containers and wrote the std::vector and std::map analogues. Inside, these analogues have std::array that can work in constexpr. Proposal [P0784] Standard containers and constexpr studies the possibility of inputting STL containers in constexpr evaluations. Note. It's important to know what an allocator is. STL containers work with memory through it. What kind of an allocator — is specified through the tempte argument. If you want to get into the topic, read this article.

Quick Q: Can computing the length of a C string really be compile-time constexpr? -- StackOverflow

Quick A: Yes, when the string being traversed is itself a constant expression, such as a string literal.

Recently on StackOverflow:

Computing length of a C string at compile time. Is this really a constexpr?

I'm trying to compute the length of a string literal at compile time. To do so I'm using following code:

#include <cstdio>

int constexpr length(const char* str)
    return *str ? 1 + length(str + 1) : 0;

int main()
    printf("%d %d", length("abcd"), length("abcdefgh"));

Everything works as expected, the program prints 4 and 8. The assembly code generated by clang shows that the results are computed at compile time:

0x100000f5e:  leaq   0x35(%rip), %rdi          ; "%d %d"
0x100000f65:  movl   $0x4, %esi
0x100000f6a:  movl   $0x8, %edx
0x100000f6f:  xorl   %eax, %eax
0x100000f71:  callq  0x100000f7a               ; symbol stub for: printf

My question: is it guaranteed by the standard that length function will be evaluated compile time?

If this is true the door for compile time string literals computations just opened for me... for example I can compute hashes at compile time and many more...