[C++] Moving small strings can invalidate data pointers

Heap allocated structures can generally be moved without invalidating pointers to the underlying data however this doesn’t always hold true with std::string.

If strings are small enough the character data can be held in the storage reserved for the class members instead of allocating on the heap. This is called small string optimisation¹. If our string’s data is stored internally then moving the object will naturally change the data’s address and invalidate any views to the data.

I encountered this issue when I used std::vector<std::string>. When the vector ran out of room and reallocated, many of my std::string_view instances became invalid and crashed the program.

This example shows the behaviour².

#include <print>
#include <string>

int main() {
    std::string small_string{"foo"};
    auto* ss_ptr0{small_string.data()};

    auto moved_ss{std::move(small_string)};
    auto* ss_ptr1{moved_ss.data()};

    std::string large_string{"fooooooooooooooooooooooooooooooooooooooooooooooooooooo"};
    auto* ls_ptr0{large_string.data()};

    auto moved_ls{std::move(large_string)};
    auto* ls_ptr1{moved_ls.data()};

    std::print("Small string addresses are equal: {}\n", ss_ptr0 == ss_ptr1);
    std::print("Large string addresses are equal: {}\n", ls_ptr0 == ls_ptr1);

    return 0;
}

We can see that the addresses are not equal for small strings.

Small string addresses are equal: false
Large string addresses are equal: true

A simple (though inefficient) solution is to simply reserve enough characters in the string to force heap allocation.