CANT

Constexpr allocations (non transient)

---

C++20-compatible library which implements constexpr non-transient allocator for standard containers.

Now it supports std::string and std::vector (just because there is no other dynamic constexpr containers in standard library)

Compilers support:

• Clang >= 12.0.0
• MSVC >= 19.31
• GCC now is not supported (due to bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115233)

Now there is convinient way to construct strings and vectors at compile time:

constexpr std::string_view str1 = "It is string which was builded ";
constexpr std::string_view str2 = "as concatenation of two strings";

constexpr auto concat_result = 
    cant::too_constexpr(
        []() -> std::string
        {
            return std::string(str1) + std::string(str2);
        }
);

// It does work!
static_assert(concat_result == "It is string which was builded as concatenation of two strings", "error");

How to use

It's necessary to pass functional object (lambda, for example) which produce result to function cant::too_constexpr.

Limitations

• Lamda passed to cant::too_constexpr must NOT:

  • have any arguments (arguments with default values is possible)
  • capture any objects

• It should use only other constexpr objects.

• Nested containers e.g. std::vector<std::string>, now is not supported.

Build examples

mkdir build
cd build
cmake ..
cmake --build .

Examples require C++23 standard, because it contain example with std::unordered_map.

Other examples can work with C++20.

Possible errors

---

Clang

If your constexpr object is too big and you get compile error which looks like ...constexpr evaluation hit maximum step limit..., just increase constexpr steps by adding flat -fconstexpr-steps=N.

Future

• support nested containers
• improve compile time