C ++模板语法

Question

I don't understand what is wrong with this code.

gcc reports "Client.h:29: error: expected template-name before '<' token"

As far as I'm aware I'm followed the template syntax correctly, but it could be that the error message is confusing me and is not the problem

client.h

class Client : public BaseDll<DllClient> [line 29]
{
..snip..
};

basedll.h

template<typename T>
class BaseDll : public Base
{
public:
    ..snip..

private:
  T* _dll;
};

Answer 1

class Base{
};
template<typename T>
class BaseDll:public Base{
public:
private:
T* _dll;
};
class DllClient{
};
class Clien:public BaseDll<DllClient>
{
};

this compiled for me without problems, so I don't think the problem lies within what you posted. My best bet would be that you made a syntax error in client.h, maybe something as simple as forgetting a semicolon after another class definition or some macro that's messing with your code

Answer 2

I'm so sorry everyone, a school-boy error has been made, BaseDll is declared in another namespace. As soon as I added the namespace qualifier, the problem has gone.

Answer 3

也许这只是一个简单的问题：您是否在client.h包含了basedll.h ？

Answer 4

to cover the basics:

does client-h #include basedll.h? do they user different include guards?

Further troubleshooting: does it work with a non-template base class? does it work then you typedef the template instaltiation:

typedef BaseDll<DllClient> tClientBase;
class Client : public tClientBase { ... }

[edit] OK, next:

if you put the following two lines directly under the BaseDll declaration:

template <typename T>
class BaseDll
{ ... 
};
class DummyFoo;
typedef BaseDll<DummyFoo> tDummyFoo;

Answer 5

I think you snipped away the problem. Are you including something to define 'DllClient' ?

Answer 6

A possible cause of this is that there is an inter-dependency between the different header files:

// client.h
#ifndef CLIENT
#define CLIENT

#include "base.h"

// ... 

class Client : public BaseDll<DllClient>
{
  // ..snip..
};

#endif


// base.h
#ifndef BASE
#define BASE

#include "client.h"

template<typename T>
class BaseDll : public Base
{
public:
  // ..snip..

private:
  T* _dll;
};

#endif

Now imagine we're parsing 'base.cpp' then the preprocessor will do the following:

#include "base.h"
#ifndef BASE        <--- BASE unset, keep going
#define BASE
#include "client.h"
#ifndef CLIENT
#define CLIENT
#include "base.h" 
#ifndef BASE        <--- BASE set, skip base.h, return to client.h
class client
 : public BaseDll<DllClient>  <-- ERROR, BaseDll not defined.

If this is the problem, then you potentially can get around it by forward declaring the base template in client.h:

// client.h
#ifndef CLIENT
#define CLIENT

// #include "base.h"    <-- remove include
template <typename DLL_CLIENT>
class BaseDll;

// ... 

class Client : public BaseDll<DllClient>
{
  // ..snip..
};

#endif