引用或指向不完整类型的std :: vector

Question

As answered here: How can an incomplete type be used as a template parameter to vector here? usage of incomplete type as template argument when instantiating a template component can result in undefined behaviour. But does that rule hold true when we have only pointer/reference to template component with incomplete type as argument? Does the instatiation happen in this case too?

For example:

// SomeAlgoInterface.hpp

#include <vector> 

struct Result; // forward declaration

class SomeAlgoInterface
{
  public:

   virtual ~SomeAlgoInterface() = default; 

  public:

    // the following line is definitely OK, ...
    virtual void f1(const Result & result) = 0; 

    // ... but I'm not quite sure about the following one
    virtual void f2(const std::vector<Result> & results) = 0; 
};

In other words, is the code above valid or not?

Answer 1

The declaration is correct, so long as you don't call f2 .

The compiler doesn't need to know the internal of Result class if you don't call f2 . There's no storage allocation in the declaration.

If some compilation unit call f2 , you need to supply the complete type for Result class, or you need another reference parameter to call f2 :

void another_f(SomeAlgoInterface& i, std::vector<Result>& results)
{
    i.f2(results);  
}