C++ Packet Builder with Templates

Question

PacketBuilder is a little Class which allow to write into a char* array. The Append Functions:

template <class T>
void PacketBuilder::Append(const T value)
{
 memcpy((&m_Buffer) + m_Index, (const void*) &value, sizeof(T));
 m_Index += sizeof(T);
}

Compiling without errors. If I call Append and use T as unsigned short (WORD). It works great. If I use T as unsigned char. I get an Linker Error.

m_Builder.Append<unsigned char>(0x01); // Error: LNK1120
m_Builder.Append<unsigned short>(0x0001); // Works

Error from VS2010 (sry i got german vs2010):

error LNK2019: Verweis auf nicht aufgelöstes externes Symbol ""public: void __thiscall PacketBuilder::Append(unsigned char)" (??$Append@E@PacketBuilder@@QAEXE@Z)" in Funktion ""public: void __thiscall Client::DoHandshake(void)" (?DoHandshake@Client@@QAEXXZ)". 1>C:\XXX\C++\SilkroadEmu\Debug\LoginServer.exe : fatal error LNK1120: 1 nicht aufgelöste externe Verweise.

Translated to English:

error LNK2019: Unresolved external symbol ""public: void __thiscall PacketBuilder::Append(unsigned char)" (??$Append@E@PacketBuilder@@QAEXE@Z)" in Function ""public: void __thiscall Client::DoHandshake(void)" (?DoHandshake@Client@@QAEXXZ)". 1>C:\XXX\C++\SilkroadEmu\Debug\LoginServer.exe : fatal error LNK1120: 1 unsresolved external symbol.

Answer 1

Your PacketBuilder is not a class template, as far as I can see. PacketBuilder::Append is however a template method, which requires that it's definition must be visible at any point of instantiation of this method. The only really safe way to assure this is to put the complete definition of this method template into the header file:

class PacketBuilder {
 // declarations of non-template members

public:
 template <class T> 
 void Append(const T value) 
 { 
  memcpy((&m_Buffer) + m_Index, (const void*) &value, sizeof(T)); 
  m_Index += sizeof(T); 
 }

};

Answer 2

Put the method definition in the header (hpp file), not in the implementation (cpp) file.