C++: error: need 'typename' before 'T::iterator' because 'T' is a dependent scope

Meaning

When the compiler parses a template it does not yet know what T is, so T::value_type is assumed to be a value. Prefix it with typename. The same idea applies to calling member templates, which need .template / ->template.

Common causes

• Using T::iterator, T::value_type or std::vector<T>::const_iterator as a type inside a template without typename
• Calling a member template on a dependent object without .template
• Using a dependent base class member without this->

Detailed fix by platform

Code examples

#include <vector>

template <typename C>
void show(const C& c) {
    // C::const_iterator it = c.begin();        // error: need 'typename' before 'C::const_iterator'
    typename C::const_iterator it = c.begin();  // OK
    for (; it != c.end(); ++it) { /* ... */ }
}

int main() { show(std::vector<int>{1, 2, 3}); }

How to diagnose

1. Template — Is the name inside a template?
2. Kind — Is the dependent name a type?
3. Base — Is the member from a dependent base class?

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