Я реализовал шаблон класса writer.writer имеет переменную-член шаблона s_.Тип s_ равен Stream.writer ожидает, что Stream имеет функцию-член, которая может вызываться с параметрами const char* buf и size_t len.
Вот первая версия writer:
// clang++ -Wconversion test.cpp
#include <cstddef>
#include <ostream>
template <class F> struct size_arg_type;
template <typename Ret, typename Cls, typename T1, typename T2>
struct size_arg_type<Ret (Cls::*)(T1, T2)> {
using type = T2;
};
template <typename Stream>
struct writer {
writer(Stream& s):s_(s) {}
void write(const char* buf, size_t len) {
// The type of 2nd parameter depends on Stream
s_.write(buf, len);
}
Stream& s_;
};
struct user_stream1 {
void write(const char*, size_t) {}
};
struct user_stream2 {
void write(const char*, std::streamsize) {}
};
struct user_stream3 {
void write(const char*, size_t) {}
void write() {}
};
#include <sstream>
int main() {
{ // size_type is size_t
user_stream1 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
{ // size_type is std::streamsize
user_stream2 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
#if 1
{ // size_type is size_t but has overloaded member function
user_stream3 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
#endif
{ // size_type is std::streamsize return type is std::ostream&
std::stringstream s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
}
Запуск демо: https://wandbox.org/permlink/JtEHDG3plWxe4vwB
Если я установил -Wconversion flags на clang ++, я получил следующие предупреждения:
clang++ -std=c++17 -Wconversion test.cpp
test.cpp:18:23: warning: implicit conversion changes signedness: 'size_t' (aka 'unsigned long') to
'std::streamsize' (aka 'long') [-Wsign-conversion]
s_.write(buf, len);
~~~~~ ^~~
test.cpp:50:11: note: in instantiation of member function 'writer<user_stream2>::write' requested here
w.write(buf, sizeof(buf));
^
test.cpp:18:23: warning: implicit conversion changes signedness: 'size_t' (aka 'unsigned long') to
'std::streamsize' (aka 'long') [-Wsign-conversion]
s_.write(buf, len);
~~~~~ ^~~
test.cpp:64:11: note: in instantiation of member function 'writer<std::__cxx11::basic_stringstream<char>
>::write' requested here
w.write(buf, sizeof(buf));
^
2 warnings generated.
Compilation finished at Wed May 1 09:37:37
Я попытался найти способ подавления предупреждений без прагмы.Я придумал подход static_cast.Чтобы сделать static_cast, мне нужно знать 2-й тип параметра.
Поэтому я реализовал некоторый экстрактор типа параметра:
// clang++ -std=c++17 -Wconversion test.cpp
#include <cstddef>
#include <ostream>
template <class F> struct size_arg_type;
template <typename Ret, typename Cls, typename T1, typename T2>
struct size_arg_type<Ret (Cls::*)(T1, T2)> {
using type = T2;
};
template <typename Stream>
struct writer {
writer(Stream& s):s_(s) {}
void write(const char* buf, size_t len) {
write_impl(&Stream::write, buf, len);
}
template <typename Write>
void write_impl(Write, const char* buf, size_t len)
{
s_.write(buf, static_cast<typename size_arg_type<Write>::type>(len));
}
Stream& s_;
};
struct user_stream1 {
void write(const char*, size_t) {}
};
struct user_stream2 {
void write(const char*, std::streamsize) {}
};
struct user_stream3 {
void write(const char*, size_t) {}
void write() {}
};
#include <sstream>
int main() {
{ // size_type is size_t
user_stream1 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
{ // size_type is std::streamsize
user_stream2 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
#if 1
{ // size_type is size_t but has overloaded member function
user_stream3 s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
#endif
{ // size_type is std::streamsize return type is std::ostream&
std::stringstream s;
writer w(s);
char buf[] = "123";
w.write(buf, sizeof(buf));
}
}
Он работает, как я и ожидал.Однако в случае user_stream3, в котором перегружена функция-член write(), возникает ошибка компиляции.
Запуск демонстрации: https://wandbox.org/permlink/TDPlQ3nXzIKjSlhY
Для получения определенной перегрузки функций-членов Iнужно знать тип указателя на функцию-член.Однако это непредсказуемо.
clang++ -std=c++17 -Wconversion test.cpp
test.cpp:18:9: error: no matching member function for call to 'write_impl'
write_impl(&Stream::write, buf, len);
^~~~~~~~~~
test.cpp:63:11: note: in instantiation of member function 'writer<user_stream3>::write' requested here
w.write(buf, sizeof(buf));
^
test.cpp:22:10: note: candidate template ignored: couldn't infer template argument 'Write'
void write_impl(Write, const char* buf, size_t len)
^
1 error generated.
Compilation exited abnormally with code 1 at Wed May 1 09:48:42
Есть ли какой-нибудь хороший способ узнать тип размера или подавить предупреждение без прагмы?