c ++模板特征-编译时不包含头文件

Question

How is it that this works? Might be doing something wrong here.

def.H

enum some_enum { FAKE = 0, };

template < some_enum T> struct example_trait;

trait_implementation.H

#include "def.H"

template<> struct example_trait<FAKE> {  
static constexpr size_t member_var = 3;  };

generic_alg.H

#include "def.H"
template < some_enum T, typename TT = example_trait<T> > void
function() { std::cout << TT::member_var << std::endl; }

main.C

I can run this in my main as long as I include the headers in this order

1.  #include trait_implementation.H 

2.  #include generic_alg.H 

---

int main() {
    function<FAKE>();
    return 0; 
}

How is it that this compiles? can generic_alg.H compiles with only a forward declared traits class. It can see the traits definition when included in the right order, even though generic_alg.H itself does not include the trait_implementation.H. How is that plausible?

Using an online compiler I can only re-create: https://onlinegdb.com/B1BEUlp7E

Answer 1

#include is, for most practical purposes, a request to dump the contents of the include -ed file directly into your source code.

So even though generic_alg.H uses stuff it doesn't define or include, the only thing getting directly compiled is main.C , and the complete definition from trait_implementation.H is dumped into main.C directly before generic_alg.H , so the definition exists when it gets around to compiling the code from generic_alg.H .

Even though it works, it's still a bad idea, because now every source file that uses generic_alg.H must explicitly include trait_implementation.H first, and there is no obvious documentation making that dependency clear.