[英]How do I use Templates to make aliases with `using` (making parameterized aliases) in C++?
我正在閱讀Bjarne Stroustrup的“The C ++ Programming Language”第4版。 在本書的第一部分,我發現的使用using
看起來像如下:
// Error is in these 2 lines
template<typename T>
using Iterator<T> = typename T::iterator;
*有關完整程序和錯誤消息,請參閱[**] *
這正是我在頁面105中找到的。當我把它變成一個完整的程序並試圖編譯它時, g++
給了我這個錯誤masseage:
> g++ -std=c++14 -o fnd find_all.cpp
find_all.cpp:13:15: error: expected '=' before '<' token
using Iterator<T> = typename T::iterator;
^
find_all.cpp:13:15: error: expected type-specifier before '<' token
我在這段代碼中找不到任何問題,( 我是C ++的新手,我用我的小知識找不到問題 )(更混亂的是我在Bjarne的書上發現了這個)
有人能告訴我為什么那段代碼會出錯?
注意:但是如果我用
typename C::iterator
替換Iterator<C>
(見下文),它工作正常,沒有錯誤!
[**]完整程序和錯誤消息:
// Error is in these 2 lines
template<typename T>
using Iterator<T> = typename T::iterator;
// -------------------------------------------
// For the completeness I'll include my complete program here
template<typename C, typename V>
vector<Iterator<C>> find_all(C& c, V v) // find all occurrences of v in c
{
vector<Iterator<C>> res;
for (auto p = c.begin(); p!=c.end(); ++p)
if (∗p==v)
res.push_back(p);
return res;
}
void test()
{
string m {"Mary had a little lamb"};
for (auto p : find_all(m, 'a'))
if (*p == 'a')
cerr << "string bug!\n";
list<double> ld { 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 1.1, 1.1 };
for (auto p : find_all(ld, 1.1))
if (*p == 1.1)
cerr << "list bug!\n";
vector<string> strv { "blue", "yellow", "red", "white", "orange", "blue" };
for (auto p : find_all(strv, "blue"))
if (*p == "blue")
cerr << "string vector bug!\n";
}
int main(void)
{
test();
return 0;
}
錯誤信息:
> g++ -std=c++14 -o fnd find_all.cpp
find_all.cpp:13:15: error: expected '=' before '<' token
using Iterator<T> = typename T::iterator;
^
find_all.cpp:13:15: error: expected type-specifier before '<' token
find_all.cpp:16:8: error: 'Iterator' was not declared in this scope
vector<Iterator<C>> find_all(C& c, V v)
^
find_all.cpp:16:17: error: template argument 1 is invalid
vector<Iterator<C>> find_all(C& c, V v)
^
find_all.cpp:16:17: error: template argument 2 is invalid
find_all.cpp:16:18: error: expected unqualified-id before '>' token
vector<Iterator<C>> find_all(C& c, V v)
^
find_all.cpp: In function 'void test()':
find_all.cpp:30:31: error: 'find_all' was not declared in this scope
for (auto p : find_all(m, 'a'))
^
find_all.cpp:35:32: error: 'find_all' was not declared in this scope
for (auto p : find_all(ld, 1.1))
^
find_all.cpp:40:37: error: 'find_all' was not declared in this scope
for (auto p : find_all(strv, "blue"))
首先必須省略<T>
template<typename T>
using Iterator = typename T::iterator;
定義類模板或函數模板時,使用:
template <typename T> struct Foo { };
template <typename T> T bar() { return T{}; }
定義模板時,不要使用Foo<T>
或bar<T>
。
同樣,使用模板定義別名時,您需要使用:
template <typename T>
using Iterator = typename T::iterator;
^^ Don't include <T>
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