When I was developing a new feature for Flexc++, I ran into a compiler error stating “expected initializer before ‘const’”.
I was building a function which returned an object of an inner class. However, the outer class was a class template, and the template parameter was used in the inner class:
template <typename StreamInfo> class ScannerTemplate { protected: struct BufferData { StreamInfo d_streamInfo; }; public: BufferData const switchBuffer(StreamInfo const &streamInfo); };
The implementation of switchBuffer looked like this:
template <typename StreamInfo> inline ScannerTemplate<StreamInfo>::BufferData const ScannerTemplate<StreamInfo>::switchBuffer(StreamInfo const &streamInfo) { // ... }
Why did I get the error that an initializer was expected?
After some digging, I found that this is a (deprecated) implicit typename: At compile time, the compiler does not know what the instantiation of ScannerTemplate looks like.
To turn this into an explicit typename, just prefix typename to the return type:
template <typename StreamInfo> inline typename ScannerTemplate<StreamInfo>::BufferData const ScannerTemplate<StreamInfo>::switchBuffer(StreamInfo const &streamInfo) { // ... }
For more information, look in the C++ Annotations as usual :)