我们可以在没有模板的情况下使用 C++ 关键字“typename”吗？

Question

I saw this code in a header file.

using pointIndex = typename std::pair<std::vector<double>, size_t>;

1. Can we use "typename" here without a template?
2. Is it necessary to use typename here?
3. What is the difference between "typedef" and "typename"?

Answer 1

typename and typedef are completely different things.

typedef is a keyword used to introduce a type alias (a type def inition). In recent standards you can also use using for this. So:

typedef int MyThing;

// or
using MyThing = int;

typename is a keyword that says "the next thing is a type". It's used when dealing with templates in some situations, both in template declarations (eg template <typename T> void foo() { /*..*/ } ) and to help the parser along in some situations . In the example you've given, it is valid, but redundant .

The two things are entirely different, and thus not interchangeable.