有一天你想对容器重载运算符, 但是却出现了神秘问题
在日复一日地对容器书写
for.cppview raw1
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| for (auto it = container.begin(); it != container.end();) {
std::cout << *it++;
if (it != container.end()) std::cout << ", ";
}
|
后, 你终于受不了了, 于是你决定写一个重载来一劳永逸地解决这个问题
overload.cppview raw1
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| #include <iostream>
#include <set>
#include <vector>
template <class Container,
decltype(std::declval<Container>().begin(),
std::declval<Container>().end(),
0) * = nullptr>
std::ostream &operator<<(std::ostream &os, const Container &container) {
if (container.begin() == container.end()) return os << "[]";
os << '[';
for (auto it = container.begin(); it != container.end();) {
std::cout << *it++;
if (it != container.end()) std::cout << ", ";
}
os << ']';
return os;
}
int main() {
std::vector<int> v{1, 1, 4, 5, 1, 4};
std::cout << v << std::endl;
std::set<int> s{1, 1, 4, 5, 1, 4};
std::cout << s << std::endl;
return 0;
}
|
一切看起来都是那么美好, 直到你尝试输出一个字符串. 在你输出字符串时,
编译器拒绝了你的代码, 并说你的重载和
sign.cppview raw1
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| template <class CharT, class Traits, class Allocator>
std::basic_ostream<CharT, Traits> &
operator<<(std::basic_ostream<CharT, Traits> &,
const std::basic_string<CharT, Traits, Allocator> &);
|
撞车了
为什么会这样呢? 答案在于部分模板特化的 匹配规则,
简单来说, 编译器不能确定你的重载和 <string>
里的重载哪个更特殊, 画成 Hasse 图是这样的:
注意到你的 重载 和
<string> 里的 重载 是不可比的,
所以在匹配时无法决定哪个优先级更高
因此正确的写法应该是这样:
overload2.cppview raw1
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| #include <iostream>
#include <set>
#include <vector>
template <class CharT,
class Traits,
class Container,
decltype(std::declval<Container>().begin(),
std::declval<Container>().end(),
0) * = nullptr>
std::basic_ostream<CharT, Traits> &
operator<<(std::basic_ostream<CharT, Traits> &os, const Container &container) {
if (container.begin() == container.end()) return os << "[]";
os << '[';
for (auto it = container.begin(); it != container.end();) {
std::cout << *it++;
if (it != container.end()) std::cout << ", ";
}
os << ']';
return os;
}
int main() {
std::vector<int> v{1, 1, 4, 5, 1, 4};
std::cout << v << std::endl;
std::set<int> s{1, 1, 4, 5, 1, 4};
std::cout << s << std::endl;
std::string str = "114514";
std::cout << str << std::endl;
return 0;
}
|
参考资料
