在C ++中迭代向量

Question

So based on other examples I've found, I'm led to believe that this would be the proper code to iterate over m_vect:

for(vector<T>::iterator it = m_vect.begin(); it != m_vect.end(); ++it)

However, upon attempting to compile, I get the following error on that line:

heap.h:167:6: error: need ‘typename’ before ‘std::vector<T>::iterator’ because ‘std::vector<T>’ is a dependent scope

Like I said, I copied and adapted the line from another piece of code, so I'm really not sure what I'm doing right and wrong. Any insight?

To clarify, this is in a template function, and I have declared 'template '. m_vect is of type vector. Aaaand I don't know how to display less than and greater than properly...

Answer 1

Thankfully in C++11 you can just have the compiler figure it out. The compiler already knows what m_vect is, so you can tell it:

for (auto it= m_vect.begin(); ( it != m_vect.end()) ; ++ it ) { }

But wait, there's more. In c++11 you can even just iterate over everything in m_vect

for (auto it : m_vect ) { }

Can you tell that I think iterating in C++03 was insane and I never saw anybody do it in real life, and in C++11 it is a thousand times better?

Answer 2

This appears to be in a template function, and vector<T>::iterator is a type which is dependent on T , which is a template parameter. Since templates can be specialized with unique definitions for any different type, the compiler has no way of being sure, until the template is actually instantiated, whether vector<T>::iterator is a type, or a static member. If it is a static member, this does not make any sense, syntactically:

for(vector<T>::iterator it = m_vect.begin(); it != m_vect.end(); ++it)

What you need to do is tell the compliler that vector<T>::iterator is a type. Use typename for that:

for(typename vector<T>::iterator it = m_vect.begin(); it != m_vect.end(); ++it)

If you read your error, you can see that this is exactly what your compiler is telling you.

Answer 3

T needs to be an actual type. It should be the type of whatever is stored in the vector. Your for loop looks correct. You declared m_vect somewhere. The iterator needs the same time as whatever you declared m_vect (plus ::iterator of course).

Answer 4

This is something compilers are enforcing better now.

When you are using something derived from a template, you have to flag if it's being used as a type by adding typename . Then if the compiler knows that (for some insane reason) vector<T>::iterator is a method, it can immediately flag the error instead of heading into undefined behavior.

So just add typename like the message says:

for(typename vector<T>::iterator it = m_vect.begin(); it != m_vect.end(); ++it)

Answer 5

If you have amodern enough compiler, you can replace vector<T>::iterator with auto .

Else, find the correct type to replace the T with for your vector.