C ++如何知道容器类有push_back函数？

Question

When I look at std::back_insert_iterator http://en.cppreference.com/w/cpp/iterator/back_insert_iterator

It says the container's push_back method will be called. How does it know if the container has a method of push_back? Does it require a class that extends any virtual class and where is it defined?

Answer 1

It says the container's push_back method will be called. How does it know if the container has a method of push_back? Does it require a class that extends any virtual class and where is it defined?

No, push_back() is usually not a virtual method.

The std::back_insert_iterator is a template, which is going to call push_back() on the passed object. It the method is missing, you are going to get a compile error.

Answer 2

It doesn't magically know this to be true. Instead, it fails to compile if it doesn't find such a function.

The operations (assignment in this case) which should call push_back are written using push_back and if the given container does not have a function with that name, the invocation of that function simply won't compile.

Answer 3

How does it know if the container has a method of push_back?

It doesn't. You'll get a compile error if you try to use it on a container that doesn't have push_back .

Does it require a class that extends any virtual class and where is it defined?

No, it just requires a class that defines it. In general, templates don't use dynamic polymorphism via virtual functions; they just require the code they contain (like container.push_back(thing) ) to be valid after argument substitution.

Answer 4

If you're talking about templates (the container sent as an template) then the compiler will try to expand the template and if it doesn't find the method, it will simply fail. The templates are checked at compile time, so the compile will inform you that a container doesn't have the method you requested...

Answer 5

It doesn't know, you should know it. You have to pass the container which has push_back to std::back_insert_iterator . Otherwise, it will cause to compile errors.

For example, if you use it for a set you will get this error:

'class std::set' has no member named 'push_back'

Answer 6

No, it does not need to extend a class.

std::back_insert_iterator is a template; and the compiler will find out if the type you're trying to use the template on has this method or not when he instantiates it.

I think this type of requiring a certain interface via templates is called an Implicit Interface (although I can't find a definitive reference at this point) - in contrast to explicit interfaces exhibited by deriving from a certain (possibly abstract) class.

Answer 7

It is a templatised class.

It will generate the code at compile time. In the compile time, it will look for push_back function in container. If there is not push_back , it will not compile.