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[英]Code compiles with g++ but not clang++ if incomplete type is used in templated function
[英]std::function of a value templated method compiles with clang and g++ but not with msvc
以下代碼使用clang v5.0.0和g ++ v8.1.0進行 編譯,但對於Visual Studio(2013和2017)失敗:
#include <string>
#include <functional>
#include <iostream>
template <const char* name>
std::string fct() {
return name;
}
const char toto[] = "toto";
std::function<std::string()> fctptr = fct<toto>;
int main(){
std::cout << fctptr() << std::endl;
}
錯誤如下:
main.cpp(11): error C2440: 'initializing' : cannot convert from 'std::string (__cdecl *)(void)' to 'std::function<std::string (void)>'
1> No constructor could take the source type, or constructor overload resolution was ambiguous
我試圖用typedef將std :: function替換為函數指針,例如:
typedef std::string(*Fctptr)();
Fctptr fctptr = fct<toto>;
但是,我遇到了同樣的錯誤。
是msvc編譯器的錯誤,還是以上代碼不符合標准?
FWIW,以下使用g ++ 6.4.0( g++ -std=c++11
)無法編譯。
#include <string>
#include <functional>
#include <iostream>
template <const char* name>
std::string fct() {
return name;
}
const char toto[] = "toto";
std::function<std::string()> fctptr = fct<toto>;
int main(){
std::cout << fctptr() << std::endl;
}
這是錯誤消息:
socc.cc:11:43: error: the value of ‘toto’ is not usable in a constant expression
std::function<std::string()> fctptr = fct<toto>;
^~~~
socc.cc:10:12: note: ‘toto’ was not declared ‘constexpr’
const char toto[] = "toto";
將toto
的定義更改為
constexpr char toto[] = "toto";
解決了問題。
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