An approach for runtime detection inside a constexpr function
C++ CMake
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About in_constexpr

An approach of detecting if inside a constexpr context in a constexpr function.

By being able to detect if we're within a constexpr context we can choose to implement a runtime specific algorithm while having a different algorithm for doing something at compile time.

The approach is discussed in this post


  • if (in_constexpr()/in_runtime()) - Being able to detect if in runtime or compile time
  • smart_assert - constexpr friendly assert


  • Only tested on GCC 5+ and Clang 3.8+
  • x86 only so far
  • Runtime approach only works on linux (replacing code in the binary should work under windows for now)
  • See detailed descriptions about caveats here
  • The library must be statically linked
  • The initialization logic relies on being able to modify the .text segment of your program

Using the library

Building and installing

You can compile and install this library using cmake. You will need to link this library in as a static library. You can customize and install the library using the following:

cmake <code directory> -DCMAKE_INSTALL_PREFIX=<install directory> && make -j && make install

Then just link in as you would a normal library. You can try the examples in examples/ to see how to use the library.

Using the API and an example

To test if this library is working, you can call in_constexpr::is_setup() to see if it is working. The library should automatically call the setup by means of the constructor attribute and so you shouldn't need to explicitly call in_constexpr::initialize() but if it's not happening automatically, you can call that function.

The library provides an in_constexpr() and in_runtime() macro/method that returns if a constexpr function is within which context. This can be used to provide different code paths in each case. Note, you can't do something like if (!in_constexpr()) due to those methods being a syntatic sugar. Use either if (in_constexpr()) or if (in_runtime()).

The library also provides a smart_assert which you can use regular assert but stil work within constexpr functions.

You can find more examples under the examples folder.

const int N_MAX            = 30;
int factorial_cache[N_MAX] = {0};

constexpr int factorial(int n) {
  smart_assert(n >= 0 && n < N_MAX, "N >= 0 && N <= N_MAX");

  if (n == 0)
    return 1;
  else {
    if (in_constexpr()) {
      return n * factorial(n - 1);
    } else {
      std::cout << "Calling factorial " << n << std::endl;
      // Since we're in runtime, we can cache results.
      if (factorial_cache[n] == 0) {
        factorial_cache[n] = n * factorial(n - 1);
      return factorial_cache[n];

int main() {
  volatile int a = 5;
  volatile int b = 6;
  std::cout << factorial(a) << std::endl;
  std::cout << factorial(b) << std::endl;

  constexpr int c = factorial(3);
  // Compiler error!
  // constexpr int d = factorial(-5);

  std::cout << c << std::endl;
  // std::cout << factorial(d) << std::endl;
  return 0;

Will produce the following outputs

Calling factorial 5
Calling factorial 4
Calling factorial 3
Calling factorial 2
Calling factorial 1
Calling factorial 6
Calling factorial 5