My primary goal is to avoid dynamic memory allocation.
For example, can I be sure which std::string methods will / will not allocate new heap memory?
Is there a way to disallow new allocations by a std::string instance?
Is there a standard fixed length string class?
For example, can I be sure which std::string methods will / will not allocate new heap memory?
No, the standard doesn't guaratee non-allocation.
However, you can provide a user defined allocator to std::basic_string. If you don't use dynamic allocation in your custom allocator, then there will be no dynamic allocation.
Is there a standard fixed length string class?
No.
Is there a way to disallow new allocations by a std::string instance?
No, but instead we can use std::pmr::string since c++17 (If the string is guaranteed to be small enough: say shorter than 16bytes, we may benefit
from the std::string's short string optimization in recent version STLS then memory allocation would not happen, but it's a black-box)
constexpr size_t kMaxLen = 256;
char buffer[kMaxLen] = {};
std::pmr::monotonic_buffer_resource pool{std::data(buffer),std::size(buffer)};
std::pmr::string vec{&pool};
// assert that we don't have a string with a length larger than kMaxLen - 1, or we will trigger heap memory allocations
Is there a standard fixed-length string class?
NO, but we may choose non standard libraries as the alternatives: