Skip to content

Compile errors

本章总结与dependent name lookup相关的compile error。

error: expected primary-expression before ‘>’ token

这是一种非常常见的compile错误，在 "thegreenplace Dependent name lookup for C++ templates # "Disambiguating dependent template names"" 段中介绍了这种错误，下面再补充一些例子:

1) stackoverflow C++ template compilation error: expected primary-expression before ‘>’ token

A

You need to do:

std::cout << pt.template get<std::string>("path");

Use template in the same situation as typename, except for template members instead of types.

(That is, since pt::get is a template member dependent on a template parameter, you need to tell the compiler it's a template.)

error: there are no arguments to ‘***’ that depend on a template parameter, so a declaration of ‘***’ must be available

这是一种常见的compiler错误，在"thegreenplace Dependent name lookup for C++ templates # "A simple problem and a solution""中介绍了这种错误，下面再补充一些例子:

stackoverflow Inheritance and templates in C++ - why are inherited members invisible?

template <int a>
class Test {
public:
    Test() {}
    int MyMethod1() { return a; }
};

template <int b>
class Another : public Test<b>
{
public:
    Another() {}
    void MyMethod2() {
        MyMethod1();
    }
};

int main()
{
    Another<5> a;
    a.MyMethod1();
    a.MyMethod2();
}
// g++ test.cpp

编译报错如下: 

test.cpp: In member function ‘void Another<b>::MyMethod2()’:
test.cpp:14:19: error: there are no arguments to ‘MyMethod1’ that depend on a template parameter, so a declaration of ‘MyMethod1’ must be available [-fpermissive]
         MyMethod1();
                   ^
test.cpp:14:19: note: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)

A

template <int a>
class Test {
public:
    Test() {}
    int MyMethod1() { return a; }
};

template <int b>
class Another : public Test<b>
{
public:
    Another() {}
    void MyMethod2() {
        Test<b>::MyMethod1();
    }
};

int main()
{
    Another<5> a;
    a.MyMethod1();
    a.MyMethod2();
}
// g++ test.cpp

error: ‘***’ does not name a type

这是一种常见的compiler错误，在thegreenplace Dependent name lookup for C++ templates # "Disambiguating dependent type names"中介绍了这种错误，下面再补充一些例子:

stackoverflow Propagating 'typedef' from based to derived class for 'template'

#include <iostream>
#include <vector>

template<typename T>
class A
{
public:
    typedef std::vector<T> Vec_t;
};

template<typename T>
class B: public A<T>
{
private:
    Vec_t v;  // fails - Vec_t is not recognized
};
int main()
{
    B<int> b;
}
// g++ test.cpp

编译报错如下:

test.cpp:15:2: error: ‘Vec_t’ does not name a type
  Vec_t v;  // fails - Vec_t is not recognized
  ^
test.cpp:15:2: note: (perhaps ‘typename A<T>::Vec_t’ was intended)

正确写法:

#include <iostream>
#include <vector>

template<typename T>
class A
{
public:
    typedef std::vector<T> Vec_t;
};

template<typename T>
class B: public A<T>
{
private:
    typename A<T>::Vec_t v; 
};
int main()
{
    B<int> b;
}
// g++ test.cpp

error: qualified name refers into a specialization of function template '***'

error: expected ‘;’ before ‘::’ token

下面是我写的code，在进行编译的时候，报了如下错误:

struct GenericPackerFactory  
{};

template<typename ServiceTrait, typename ...Args>
  auto GenericPackerFactory(int UserRequestID, Args &&...args) -> decltype(GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...))
  {
    static constexpr FuncNoType FuncNo = ServiceTrait::FuncNo; // 功能号
    auto Packer = GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);
    int ApiRequestID = RequestIdConverter.ToApiRequestId(UserRequestID);
    Packer.SetFuncNo(FuncNo).SetSenderID(ApiRequestID);
    // 打印通讯日志
    {
      LOG_MSG_INFO("{}的请求:", FuncNo);
      LOG_PACKER_INFO(&Packer);
    }
    Packer.SetFuncNo(FuncNo).SetSenderID(ApiRequestID);
    PackHead(Packer, ApiRequestID);
    Packer.Finish();
    return Packer;
  }

clang编译报错:

./../common/api_framework/ldp_api.h:519:51: error: qualified name refers into a specialization of function template 'GenericPackerFactory'
                auto Packer = GenericPackerFactory<ServiceTrait>::template New(std::forward<Args>(args)...);

g++编译报错:

错误：expected ‘;’ before ‘::’ token

模拟程序

下面是简化的模拟程序:

#include <type_traits>
#include <utility>
#include <iostream>

template<typename ServiceTrait>
struct GenericPackerFactory
{
    template<typename Dummy = ServiceTrait, typename ...Args>
    static auto New(Args &&...args) -> typename std::enable_if<Dummy::has_member, bool>::type
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return true;
    }
    template<typename Dummy = ServiceTrait, typename ...Args>
    static auto New(Args &&...args) -> typename std::enable_if<!Dummy::has_member,bool>::type
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return false;
    }
};
struct TestServiceTrait
{
    static constexpr bool has_member = true;
};

struct TestServiceTrait2
{
    static constexpr bool has_member = false;
};
template<typename UstTag>
class UstApi
{
public:
    template<typename ServiceTrait, typename ...Args>
    auto GenericPackerFactory(int UserRequestID, Args &&...args) -> decltype(GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...))
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);
    }
};

struct TestUstTag
{
};
int main()
{
    UstApi<TestUstTag> Api;
    Api.GenericPackerFactory<TestServiceTrait>(1, 1, 2, 3);
    Api.GenericPackerFactory<TestServiceTrait2>(1, 1, 2, 3);
}

g++ test.cpp --std=c++11编译报错如下:

test.cpp: 在成员函数‘decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...)’中:
test.cpp:38:44: 错误：expected ‘;’ before ‘::’ token
   return GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);
                                            ^
test.cpp:38:44: 错误：‘::New’未被声明
test.cpp: In instantiation of ‘decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...) [with ServiceTrait = TestServiceTrait; Args = {int, int, int}; UstTag = TestUstTag; decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) = bool]’:
test.cpp:48:55:   required from here
test.cpp:38:10: 错误：无法解析重载函数‘GenericPackerFactory’，基于向类型‘std::enable_if<true, bool>::type {aka bool}’的转换
   return GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);
          ^
test.cpp: In instantiation of ‘decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...) [with ServiceTrait = TestServiceTrait2; Args = {int, int, int}; UstTag = TestUstTag; decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) = bool]’:
test.cpp:49:56:   required from here
test.cpp:38:10: 错误：无法解析重载函数‘GenericPackerFactory’，基于向类型‘std::enable_if<true, bool>::type {aka bool}’的转换

clang test.cpp --std=c++11编译报错如下:

test.cpp: In member function ‘decltype (GenericPackerFactory<ServiceTrait>::New((forward<Args>)(args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...)’:
test.cpp:38:44: error: expected ‘;’ before ‘::’ token
   return GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);

Google: qualified name refers into a specialization of function template

下面是有价值的文章:

code.i-harness c++ - namespace - qualified name refers into a specialization of variable template

Is this-> mandatory to access Base<T> identifiers from derived classes? (2)

This code compiles with MSVC 2015, but doesn't compile with Clang 5.0.0 (trunk 304874):

template <typename T>
struct Base
{
  T data;
};

template <typename T>
struct Derived : Base<T>
{
  auto getData() const
  {
    return data;
  }
};

粗略的浏览了一下这篇文章的内容，我想起了this dependent name、然后惊奇的发现我错误地将 UstApi::GenericPackerFactory 和 struct GenericPackerFactory 命名为相同的名字，这就导致了compiler将 GenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...); 中的GenericPackerFactory理解为UstApi::GenericPackerFactory函数，而不是 struct GenericPackerFactory 。

显然改法就是将它们的名称区分开来，下面是修改后的，可以通过的code:

正确程序

#include <type_traits>
#include <utility>
#include <iostream>

template<typename ServiceTrait>
struct CGenericPackerFactory
{
    template<typename Dummy = ServiceTrait, typename ...Args>
    static auto New(Args &&...args) -> typename std::enable_if<Dummy::has_member, bool>::type
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return true;
    }
    template<typename Dummy = ServiceTrait, typename ...Args>
    static auto New(Args &&...args) -> typename std::enable_if<!Dummy::has_member,bool>::type
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return false;
    }
};
struct TestServiceTrait
{
    static constexpr bool has_member = true;
};

struct TestServiceTrait2
{
    static constexpr bool has_member = false;
};
template<typename UstTag>
class UstApi
{
public:
    template<typename ServiceTrait, typename ...Args>
    auto GenericPackerFactory(int UserRequestID, Args &&...args) -> decltype(CGenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...))
    {
        std::cout << __PRETTY_FUNCTION__ << std::endl;
        return CGenericPackerFactory<ServiceTrait>::New(std::forward<Args>(args)...);
    }
};

struct TestUstTag
{
};
int main()
{
    UstApi<TestUstTag> Api;
    Api.GenericPackerFactory<TestServiceTrait>(1, 1, 2, 3);
    Api.GenericPackerFactory<TestServiceTrait2>(1, 1, 2, 3);
}
// g++ test.cpp --std=c++11

输出如下:

decltype (CGenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...) [with ServiceTrait = TestServiceTrait; Args = {int, int, int}; UstTag = TestUstTag; decltype (CGenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) = bool]
static typename std::enable_if<Dummy:: has_member, bool>::type CGenericPackerFactory<ServiceTrait>::New(Args&& ...) [with Dummy = TestServiceTrait; Args = {int, int, int}; ServiceTrait = TestServiceTrait; typename std::enable_if<Dummy:: has_member, bool>::type = bool]
1
decltype (CGenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) UstApi<UstTag>::GenericPackerFactory(int, Args&& ...) [with ServiceTrait = TestServiceTrait2; Args = {int, int, int}; UstTag = TestUstTag; decltype (CGenericPackerFactory<ServiceTrait>::New((forward<Args>)(UstApi::GenericPackerFactory::args)...)) = bool]
static typename std::enable_if<(! Dummy:: has_member), bool>::type CGenericPackerFactory<ServiceTrait>::New(Args&& ...) [with Dummy = TestServiceTrait2; Args = {int, int, int}; ServiceTrait = TestServiceTrait2; typename std::enable_if<(! Dummy:: has_member), bool>::type = bool]
0