C++ ternary assignment of lambda

Question

Any idea why the following snippet doesn't compile? It complains with an error "error: operands to ?: have different types"

  auto lambda1 = [&](T& arg) {
      ...
  };
  auto lambda2 = [&](T& arg) {
      ...
  };
  auto lambda = condition ? lambda1 : lambda2;

Answer 1

Individual lambdas are translated to different classes by the compiler. For example, lambda1's definition is equivalent to:

class SomeCompilerGeneratedTypeName {
public:
  SomeCompilerGeneratedTypeName(...) { // Capture all the required variables here
  }

  void operator()(T& arg) const {
    // ...
  }

private:
  // All the captured variables here ...
};

Therefore, two different types are generated by the compiler, which causes a type incompatibility for auto lambda = condition ? lambda1 : lambda2;

The following would work:

auto lambda = condition ? std::function<void(T&)>(lambda1) : std::function<void(T&)>(lambda2);

To highlight that both lambdas are indeed different types, we can use <typeinfo> from the standard library and the typeid operator. Lambdas are not polymorphic types, so the standard guarantees that the 'typeid' operator is evaluated at compile time. This shows that the following example is valid even if RTTI is disabled:

#include <iostream>
#include <typeinfo>

int main()
{
    struct T {

    };

    auto lambda1 = [&](T& arg) {
        return;
    };

    auto lambda2 = [&](T& arg) {
      return;
    };

    std::cout << typeid(lambda1).name() << "/" << typeid(lambda1).hash_code() << std::endl;
    std::cout << typeid(lambda2).name() << "/" << typeid(lambda2).hash_code() << std::endl;

    return 0;
}

The output of the program is (with GCC 8.3, see on Gobolt):

Z4mainEUlRZ4mainE1TE_/7654536205164302515
Z4mainEUlRZ4mainE1TE0_/10614161759544824066