我有如下代码:为什么decltype在这里工作,但不是汽车?
template <typename T, typename sepT = char>
void print2d(const T &data, sepT sep = ',') {
for(auto i = std::begin(data); i < std::end(data); ++i) {
decltype(*i) tmp = *i;
for(auto j = std::begin(tmp); j < std::end(tmp); ++j) {
std::cout << *j << sep;
}
std::cout << std::endl;
}
}
int main(){
std::vector<std::vector<int> > v = {{11}, {2,3}, {33,44,55}};
print2d(v);
int arr[2][2] = {{1,2},{3,4}};
print2d(arr);
return 0;
}
如果我改变decltype
到auto
,它不会编译和抱怨(部分错误):
2d_iterator.cpp: In instantiation of ‘void print2d(const T&, sepT) [with T = int [2][2]; sepT = char]’:
2d_iterator.cpp:21:21: required from here
2d_iterator.cpp:9:36: error: no matching function for call to ‘begin(const int*&)’
2d_iterator.cpp:9:36: note: candidates are:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/string:53:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/locale_classes.h:42,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/ios_base.h:43,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/ios:43,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/ostream:40,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/iterator:64,
这究竟是为什么?
'decltype'产量' int(&)[2]',而普通的'auto'强制指针转换(与模板参数推导相同的规则)。只需使用'auto&'。 – Xeo
@Xeo这应该是一个答案。 – Angew
@Xeo你能回答吗?你也可以指出一个解释这个扣除规则的URL吗? – texasbruce