C ++-具有std :: string的统一初始化程序 [英] C++ - Uniform initializer with std::string
问题描述
我正在尝试使用C ++的字符串类的统一初始化器.下面是代码:
I am trying the uniform intializer with the string class of C++. Below is the code:
#include <iostream>
#include <string>
using namespace std;
int main()
{
string str1 {"aaaaa"};
string str2 {5, 'a'};
string str3 (5, 'a');
cout << "str1: " << str1 << endl;
cout << "str2: " << str2 << endl;
cout << "str3: " << str3 << endl;
return 0;
}
输出为:
str1: aaaaa
str2: a
str3: aaaaa
这使我挠头.为什么str2
无法获得str3
的预期结果?
This made me scratched my head. Why str2
cannot achieved the desired result as str3
?
推荐答案
initializer_list
参数的构造函数.
std::string
has a constructor that takes an initializer_list
argument.
basic_string( std::initializer_list<CharT> init,
const Allocator& alloc = Allocator() );
当您使用 braced-init-list 构造std::string
时,该构造函数始终优先.仅当 braced-init-list 中的元素不能转换为initializer_list
中的元素类型时,才考虑其他构造函数. [over.match.list]/1 中对此进行了提及.
That constructor always gets precedence when you use a braced-init-list to construct std::string
. The other constructors are only considered if the elements in the braced-init-list are not convertible to the type of elements in the initializer_list
. This is mentioned in [over.match.list]/1.
最初,候选函数是类
T
的初始化器列表构造函数([dcl.init.list]),参数列表由作为单个参数的初始化器列表组成.
Initially, the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class
T
and the argument list consists of the initializer list as a single argument.
在您的示例中,第一个参数5
可隐式转换为char
,因此initializer_list
构造函数是可行的,并且可以选择.
In your example, the first argument 5
is implicitly convertible to char
, so the initializer_list
constructor is viable, and it gets chosen.
如果将字符串中的每个字符都打印为int
s
This is evident if you print each character in the strings as int
s
void print(char const *prefix, string& s)
{
cout << prefix << s << ", size " << s.size() << ": ";
for(int c : s) cout << c << ' ';
cout << '\n';
}
string str1 {"aaaaa"};
string str2 {5, 'a'};
string str3 (5, 'a');
print("str1: ", str1);
print("str2: ", str2);
print("str3: ", str3);
输出:
str1: aaaaa, size 5: 97 97 97 97 97
str2: a, size 2: 5 97
str3: aaaaa, size 5: 97 97 97 97 97
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