Use of type dependent names must be prefixed with 'typename'

Project scenario ：

For testing purposes , Write a general iterator print template , as follows ：

template<class Con>
void PrintContainer(const Con& x)
{
    
	Con::const_iterator it = x.begin();
	while (it != x.end())
	{
    
		cout << *it << " ";
		++it;
	}
	cout << endl;
}

Problem description

I thought there was no problem , But after running, the following error is reported ：

error C7510 The dependent name must be used with “typename” The prefix

Cause analysis ：

First , Error message theory const_iterator This type is a dependent name , And the type dependent name should be typename The prefix . What is a subordinate name ？

Subordinate Name ： template The name that appears in , Depend on if template Parameters in , Then it is a subordinate Name . And if it is nested in template parameters at the same time , Then it is a nested subordinate Name .

Take this question as an example ,const_iterator Is the name that appears in the template , At the same time, it depends on the template parameters Con, meanwhile const_iterator Nested in template parameters , therefore const_iterator Is a nested subordinate Name .

Non dependent names are names that do not depend on any template parameters .

Nested subordinate names can cause some problems ：

template<typename Con>
void PrintContainer(const Con& c)
{
    
	Con::const_iterator* x;
}

For the above code , If const_iterator It's a class , be x It's a point const_iterator The pointer to , But if const_iterator Is the name of a variable , It's a multiplication action .

So in C++ There is a parsing rule in ： If the parser is template Encountered a nested subordinate name in , Then assume that the name is not a type , Unless by adding keywords typename To tell it's a type .

Solution ：

We can increase typename keyword , tell C++ Parser ,const_iterator Is a type .

template<class Con>
void PrintContainer(const Con& x)
{
    
	typename Con::const_iterator it = x.begin();
	while (it != x.end())
	{
    
		cout << *it << " ";
		++it;
	}
	cout << endl;
}

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