我正在编写一些c ++模板代码来替换当前源代码中的if-else条件。在这里,我基于两个条件派生Helper数据类型:1. isAdvice 2. SimpleOrComplex
参见以下模板代码:
template<bool isAdvice, class SH, class CH>
class IfThenElse;
template<class SH, class CH>
class IfThenElse<true, SH, CH>
{
public:
typedef SH Helper;
};
template<class SH, class CH>
class IfThenElse<false, SH, CH>
{
public:
typedef CH Helper;
};
template <bool isAdvice, bool SimpleOrComplex>
class DeriveHelper
{
public:
typedef typename IfThenElse<isAdvice,
IfThenElse<SimpleOrComplex, SimpleHelper, ComplexHelper>::Helper,
IfThenElse<SimpleOrComplex, SimpleNoAdvHelper, ComplexNoAdvHelper>::Helper>::Helper DerivedHelper;
};
但是,在编译时出现此错误:
template.cpp:135: error: type/value mismatch at argument 2 in template parameter list for 'template<bool isTradeAdvice, class SH, class GH> struct IfThenElse'
template.cpp:135: error: expected a type, got 'IfThenElse::Helper'
template.cpp:135: error: type/value mismatch at argument 3 in template parameter list for 'template<bool isTradeAdvice, class SH, class GH> struct IfThenElse'
template.cpp:135: error: expected a type, got 'IfThenElse::Helper'
有人可以提出原因吗?
您应该像为第一个typename
模板类型参数IfThenElse
那样为两个模板类型参数添加关键字
template <bool isAdvice, bool SimpleOrComplex>
class DeriveHelper
{
public:
typedef typename IfThenElse<isAdvice,
typename IfThenElse<SimpleOrComplex, SimpleHelper, ComplexHelper>::Helper,
^^^^^^^^
typename IfThenElse<SimpleOrComplex, SimpleNoAdvHelper, ComplexNoAdvHelper>::Helper>::Helper DerivedHelper;
^^^^^^^^
};
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