decltype

Use case

Write generic code that works with different binary formats (32-bit vs 64-bit).

Explanation

decltype(expr) gives us the type of an expression. In generic code, we often cannot know the type ahead of time (it depends on template parameters). decltype(bin.getEntryPoint()) becomes uint32_t or uint64_t depending on which binary type we pass in.

Code

// https://stackoverflow.com/questions/12084040/what-is-the-difference-between-decltype-and-auto-as-a-placeholder-type-for-varia

#include <cstdint>
#include <iostream>

struct MachO32 {
  uint32_t getEntryPoint() { return 0x1000; }
};

struct MachO64 {
  uint64_t getEntryPoint() { return 0x10001000; }
};

template <typename Binary>
auto findEntry(Binary &bin) -> decltype(bin.getEntryPoint()) {
  return bin.getEntryPoint();
}

int main() {
  MachO32 bin32;
  MachO64 bin64;

  auto entry32 = findEntry(bin32);
  auto entry64 = findEntry(bin64);

  std::cout << "32-bit entry: 0x" << std::hex << entry32 << "\n";
  std::cout << "64-bit entry: 0x" << std::hex << entry64 << "\n";

  return 0;
}

View on GitHub.

Output

$ ./src/c++11/build/decltype
32-bit entry: 0x1000
64-bit entry: 0x10001000