[英]what is the wrong with this example in the c++ book of Bjarne Stroustrup
我在bjarne stroustrup的c ++编程语言第4版中尝试了该示例:
#include <iostream>
#include <string>
#include <vector>
#include <list>
using namespace std;
template<typename T>
using Iterator<T> = typename T::iterator;
template<typename C, typename V>
vector<Iterator<C>> find_all(C& c, V v)
{
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";
// p is a str ing::iterator
list<double> ld {1.1, 2.2, 3.3, 1.1};
for (auto p : find_all(ld,1.1))
if (*p!=1.1)
cerr << "list bug!\n";
vector<string> vs { "red", "blue", "green", "green", "orange", "green" };
for (auto p : find_all(vs,"green"))
if (*p!="green")
cerr << "vector bug!\n";
for (auto p : find_all(vs,"green"))
*p = "ver t";
}
int main()
{
test();
return 0;
}
第4.5.1节
我在ubuntu上使用以下代码编译代码:g ++ test.cpp -o test -g -std = c ++ 11,但是出现以下错误:
iterator_of_iterator_version_2.cpp:9:15: error: expected ‘=’ before ‘<’ token
using Iterator<T> = typename T::iterator;
^
iterator_of_iterator_version_2.cpp:9:15: error: expected type-specifier before ‘<’ token
iterator_of_iterator_version_2.cpp:12:8: error: ‘Iterator’ was not declared in this scope
vector<Iterator<C>> find_all(C& c, V v)
^
iterator_of_iterator_version_2.cpp:12:17: error: template argument 1 is invalid
vector<Iterator<C>> find_all(C& c, V v)
^
iterator_of_iterator_version_2.cpp:12:17: error: template argument 2 is invalid
iterator_of_iterator_version_2.cpp:12:18: error: expected unqualified-id before ‘>’ token
vector<Iterator<C>> find_all(C& c, V v)
^
iterator_of_iterator_version_2.cpp: In function ‘void test()’:
iterator_of_iterator_version_2.cpp:24:30: error: ‘find_all’ was not declared in this scope
for (auto p : find_all(m,'a'))
^
iterator_of_iterator_version_2.cpp:29:31: error: ‘find_all’ was not declared in this scope
for (auto p : find_all(ld,1.1))
^
iterator_of_iterator_version_2.cpp:33:35: error: ‘find_all’ was not declared in this scope
for (auto p : find_all(vs,"green"))
^
iterator_of_iterator_version_2.cpp:36:35: error: ‘find_all’ was not declared in this scope
for (auto p : find_all(vs,"green"))
^
那是什么问题呢?
在这段代码中似乎找不到语法错误,因为我只是复制并粘贴了该书中的示例。
从改变开始
template<typename T>
using Iterator<T> = typename T::iterator;
至
template<typename T>
using Iterator = typename T::iterator;
使用可能相同的编译器设置在我的ubuntu 16.04上工作
为什么会这样呢? 我对此没有100%的信心,请其他人验证。
写作
using Iterator<T>
是无效的,因为在这里没有意义。 我们希望Iterator是一个模板化typedef,它将要求其parameter-type为其通用迭代器类型。 Iterator<T>
将专用于模板。 例如,对于特定类型,我们更了解它:
template<>
using Iterator<MyClass> = MyClassIterator;
至少这适用于普通模板类,我认为使用也一样。
改变这个
template<typename T>
using Iterator<T> = typename T::iterator;
对此
template<typename T>
using Iterator = typename T::iterator;
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