c++ - SFINAE 返回类型重载

Question

我正在尝试对同一函数进行两次重载，称为某物。这个函数应该以另一个函数作为参数，并且它应该根据这个另一个函数的返回类型进行重载。到目前为止我有这个:

#include <iostream>
#include <type_traits>

using namespace std;

unsigned long function1() {
    return 5;
}
void function2() {
    return;
}

template <typename Algorithm, typename enable_if<is_void<decltype(Algorithm())>::value>::type* = nullptr>
void something(Algorithm a) {
    std::cout << "algorithm returns void\n";
}
template <typename Algorithm, typename enable_if<!is_void<decltype(Algorithm())>::value>::type* = nullptr>
decltype(Algorithm()) something(Algorithm a) {
    std::cout << "algorithm returns non-void: " << a() << "\n";
}

int main() {
    something(function1);
    something(function2);
}

但是，这不会编译。这是我使用 gcc 编译器得到的构建消息:

||=== Build: Debug in Library (compiler: GNU GCC Compiler) ===|
C:\Alex\Programming\C++\Library\test.cpp||In instantiation of 'decltype (Algorithm()) something(Algorithm) [with Algorithm = long unsigned int (*)(); typename std::enable_if<(! std::is_void<decltype (Algorithm())>::value), void>::type* <anonymous> = 0u; decltype (Algorithm()) = long unsigned int (*)()]':|
C:\Alex\Programming\C++\Library\test.cpp|23|required from here|
C:\Alex\Programming\C++\Library\test.cpp|20|warning: no return statement in function returning non-void [-Wreturn-type]|
C:\Alex\Programming\C++\Library\test.cpp||In instantiation of 'decltype (Algorithm()) something(Algorithm) [with Algorithm = void (*)(); typename std::enable_if<(! std::is_void<decltype (Algorithm())>::value), void>::type* <anonymous> = 0u; decltype (Algorithm()) = void (*)()]':|
C:\Alex\Programming\C++\Library\test.cpp|24|required from here|
C:\Alex\Programming\C++\Library\test.cpp|19|error: no match for 'operator<<' in 'std::operator<< <std::char_traits<char> >((* & std::cout), ((const char*)"algorithm returns non-void: ")) << a()'|
C:\Alex\Programming\C++\Library\test.cpp|19|note: candidates are:|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|106|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|106|note:   no known conversion for argument 1 from 'void' to 'std::basic_ostream<char>::__ostream_type& (*)(std::basic_ostream<char>::__ostream_type&) {aka std::basic_ostream<char>& (*)(std::basic_ostream<char>&)}'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|115|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>; std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|115|note:   no known conversion for argument 1 from 'void' to 'std::basic_ostream<char>::__ios_type& (*)(std::basic_ostream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|125|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|125|note:   no known conversion for argument 1 from 'void' to 'std::ios_base& (*)(std::ios_base&)'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|164|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|164|note:   no known conversion for argument 1 from 'void' to 'long int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|168|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|168|note:   no known conversion for argument 1 from 'void' to 'long unsigned int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|172|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|172|note:   no known conversion for argument 1 from 'void' to 'bool'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|93|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char; _Traits = std::char_traits<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|93|note:   no known conversion for argument 1 from 'void' to 'short int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|179|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|179|note:   no known conversion for argument 1 from 'void' to 'short unsigned int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|107|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|107|note:   no known conversion for argument 1 from 'void' to 'int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|190|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|190|note:   no known conversion for argument 1 from 'void' to 'unsigned int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|199|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|199|note:   no known conversion for argument 1 from 'void' to 'long long int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|203|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|203|note:   no known conversion for argument 1 from 'void' to 'long long unsigned int'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|218|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|218|note:   no known conversion for argument 1 from 'void' to 'double'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|222|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|222|note:   no known conversion for argument 1 from 'void' to 'float'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|230|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|230|note:   no known conversion for argument 1 from 'void' to 'long double'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|243|note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|243|note:   no known conversion for argument 1 from 'void' to 'const void*'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|121|note: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|121|note:   no known conversion for argument 1 from 'void' to 'std::basic_ostream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\basic_string.h|2750|note: template<class _CharT, class _Traits, class _Alloc> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const std::basic_string<_CharT, _Traits, _Alloc>&)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\basic_string.h|2750|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   mismatched types 'const std::basic_string<_CharT, _Traits, _Alloc>' and 'void'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|469|note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, _CharT)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|469|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   deduced conflicting types for parameter '_CharT' ('char' and 'void')|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|474|note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|474|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'char'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|480|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|480|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'char'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|486|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|486|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'signed char'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|491|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|491|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'unsigned char'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|511|note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const _CharT*)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|511|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   mismatched types 'const _CharT*' and 'void'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|323|note: template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\ostream.tcc|323|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'const char*'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|528|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|528|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'const char*'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|541|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|541|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'const signed char*'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|546|note: template<class _Traits> std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|546|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|note:   cannot convert 'a()' (type 'void') to type 'const unsigned char*'|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|600|note: template<class _CharT, class _Traits, class _Tp> std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&)|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|600|note:   template argument deduction/substitution failed:|
C:\Alex\Programming\C++\Library\test.cpp|19|required from 'decltype (Algorithm()) something(Algorithm) [with Algorithm = void (*)(); typename std::enable_if<(! std::is_void<decltype (Algorithm())>::value), void>::type* <anonymous> = 0u; decltype (Algorithm()) = void (*)()]'|
C:\Alex\Programming\C++\Library\test.cpp|24|required from here|
c:\program files (x86)\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\ostream|600|error: forming reference to void|
C:\Alex\Programming\C++\Library\test.cpp||In instantiation of 'decltype (Algorithm()) something(Algorithm) [with Algorithm = void (*)(); typename std::enable_if<(! std::is_void<decltype (Algorithm())>::value), void>::type* <anonymous> = 0u; decltype (Algorithm()) = void (*)()]':|
C:\Alex\Programming\C++\Library\test.cpp|24|required from here|
C:\Alex\Programming\C++\Library\test.cpp|20|warning: no return statement in function returning non-void [-Wreturn-type]|
||=== Build failed: 2 error(s), 10 warning(s) (0 minute(s), 2 second(s)) ===|

我想在 C++ 中实现什么？如果是，怎么办？

Answer 1

Algorithm 不调用该函数，而是创建该函数的推导类型的实例。您需要另一对括号才能调用它

decltype(Algorithm()())

甚至:

std::declval<Algorithm>()()

如果您的类型是具有不可访问的默认构造函数的仿函数。