Why does libc++’s implementation of std::string take up 3x memory as libstdc++?

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Here is a short program to help you explore both kinds of memory usage of std::string: stack and heap.

#include <string>
#include <new>
#include <cstdio>
#include <cstdlib>

std::size_t allocated = 0;

void* operator new (size_t sz)
{
    void* p = std::malloc(sz);
    allocated += sz;
    return p;
}

void operator delete(void* p) noexcept
{
    return std::free(p);
}

int
main()
{
    allocated = 0;
    std::string s("hi");
    std::printf("stack space = %zu, heap space = %zu, capacity = %zu\n",
     sizeof(s), allocated, s.capacity());
}

Using http://melpon.org/wandbox/ it is easy to get output for different compiler/lib combinations, for example:

gcc 4.9.1:

stack space = 8, heap space = 27, capacity = 2

gcc 5.0.0:

stack space = 32, heap space = 0, capacity = 15

clang/libc++:

stack space = 24, heap space = 0, capacity = 22

VS-2015:

stack space = 32, heap space = 0, capacity = 15

(the last line is from http://webcompiler.cloudapp.net)

The above output also shows capacity, which is a measure of how many chars the string can hold before it has to allocate a new, larger buffer from the heap. For the gcc-5.0, libc++, and VS-2015 implementations, this is a measure of the short string buffer. That is, the size buffer allocated on the stack to hold short strings, thus avoiding the more expensive heap allocation.

It appears that the libc++ implementation has the smallest (stack usage) of the short-string implementations, and yet contains the largest of the short string buffers. And if you count total memory usage (stack + heap), libc++ has the smallest total memory usage for this 2-character string among all 4 of these implementations.

It should be noted that all of these measurements were taken on 64 bit platforms. On 32 bit, the libc++ stack usage will go down to 12, and the small string buffer goes down to 10. I don’t know the behavior of the other implementations on 32 bit platforms, but you can use the above code to find out.

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