為什么我不能將gtest ValuesIn生成器與std :: map一起使用？

Question

我正在嘗試通過以下方式使用gtest在地圖的元素上運行一些測試：

typedef std::map< std::string, std::vector< std::string > > maptype;

static maptype generateMapOfElements() {...}

class MyTest : public TestWithParam< maptype::value_type > {
...
}

TEST_P(MyTest , my_first_test) { .. }

INSTANTIATE_TEST_CASE_P(test_prefix,
                        MyTest,
                        ValuesIn( generateMapOfElements() ) ); // <- problem

問題似乎是實例化pair::swap會產生問題，因為地圖的鍵是const字符串（下面的完整日志）

1>C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\utility(48): error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\xstring(912): could be 'std::basic_string<_Elem,_Traits,_Alloc> &std::basic_string<_Elem,_Traits,_Alloc>::operator =(std::basic_string<_Elem,_Traits,_Alloc> &&) throw()'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Alloc=std::allocator<char>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\xstring(969): or       'std::basic_string<_Elem,_Traits,_Alloc> &std::basic_string<_Elem,_Traits,_Alloc>::operator =(const std::basic_string<_Elem,_Traits,_Alloc> &)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Alloc=std::allocator<char>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\xstring(987): or       'std::basic_string<_Elem,_Traits,_Alloc> &std::basic_string<_Elem,_Traits,_Alloc>::operator =(const _Elem *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Alloc=std::allocator<char>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\xstring(992): or       'std::basic_string<_Elem,_Traits,_Alloc> &std::basic_string<_Elem,_Traits,_Alloc>::operator =(_Elem)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Alloc=std::allocator<char>
1>          ]
1>          while trying to match the argument list '(const std::string, const std::basic_string<_Elem,_Traits,_Alloc>)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Alloc=std::allocator<char>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\utility(56) : see reference to function template instantiation 'void std::swap<_Ty>(_Ty &,_Ty &)' being compiled
1>          with
1>          [
1>              _Ty=std::string
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\utility(107) : see reference to function template instantiation 'void std::_Swap_adl<_Ty1>(_Ty &,_Ty &)' being compiled
1>          with
1>          [
1>              _Ty1=std::string,
1>              _Ty=std::string
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\utility(104) : while compiling class template member function 'void std::pair<_Ty1,_Ty2>::swap(std::pair<_Ty1,_Ty2> &)'
1>          with
1>          [
1>              _Ty1=const std::string,
1>              _Ty2=std::vector<testing::internal::string>
1>          ]
1>          C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\utility(211) : see reference to function template instantiation 'void std::pair<_Ty1,_Ty2>::swap(std::pair<_Ty1,_Ty2> &)' being compiled
1>          with
1>          [
1>              _Ty1=const std::string,
1>              _Ty2=std::vector<testing::internal::string>
1>          ]
1>          C:\dir\my_test.cpp(251) : see reference to class template instantiation 'std::pair<_Ty1,_Ty2>' being compiled
1>          with
1>          [
1>              _Ty1=const std::string,
1>              _Ty2=std::vector<testing::internal::string>
1>          ]

Answer 1

ValuesIn函數返回一個生成器，該生成器保留值的副本，並且該副本存儲在vector 。 （為什么要存儲副本？因為通常來說，它不能依賴原始值源在測試的生命周期內保持有效。） vector的要求之一是vector的元素類型必須是可分配的（C + +03，第23.1 / 3節）。 但是，您已經注意到， map::value_type是pair<const Key, T> 。 const鍵類型可防止分配該類型。

您可以考慮返回vector<pair<string, vector<string>>>而不是生成器函數中的map<string, vector<string>> 。