我什么时候可以通过const& std :: string而不是std :: string_view? [英] When would I pass const& std::string instead of std::string_view?
问题描述
我了解使用 std :: string_view 的动机;
可以帮助避免在函数参数中进行不必要的分配。
I understand the motivation for using std::string_view;
it can help avoid unecessary allocations in function arguments.
例如:
以下程序将创建 std :: string
来自字符串文字。
这会导致不希望的动态分配,因为我们只对观察字符感兴趣。
For example:
The following program will create a std::string
from a string literal.
This causes an undesired dynamic allocation, as we are only interested observing the characters.
#include <iostream>
void* operator new(std::size_t n)
{
std::cout << "[allocating " << n << " bytes]\n";
return malloc(n);
}
void observe_string(std::string const& str){}
int main(){
observe_string("hello world"); //prints [allocating 36 bytes]
}
使用 string_view
将解决问题:
#include <iostream>
#include <experimental/string_view>
void* operator new(std::size_t n)
{
std::cout << "[allocating " << n << " bytes]\n";
return malloc(n);
}
void observe_string(std::experimental::string_view const& str){
}
int main(){
observe_string("hello world"); //prints nothing
}
此
我什么时候选择const&的std :: string?
看看 std :: string_view
的接口,它代替了函数参数的string_view?似乎我可以替换所有由 const&
传递的 std :: string
实例。有什么反例吗?
Looking at the interface of std::string_view
, it looks as though I could replace all instances of std::string
that are passed by const&
. Are there any counter examples to this? Is std::string_view
meant to replace std::string const&
for parameter passing?
推荐答案
我何时选择
std :: string
c $ c> const& 而不是string_view
作为函数参数?
When would I choose
std::string
byconst&
instead ofstring_view
for function arguments?
您是否需要以空值结尾的字符串?如果是这样,则应该使用 std :: string const&
来保证。 string_view
不会-只是 const char
的范围。
Do you need a null-terminated string? If so, then you should use std::string const&
which gives you that guarantee. string_view
does not - it's simply a range of const char
.
如果不需要需要以null结尾的字符串,并且不需要获取数据所有权,则应使用 string_view
。如果确实需要获取数据所有权,则按值 string
可能比 string_view
。
If you do not need a null-terminated string, and you do not need to take ownership of the data, then you should use string_view
. If you do need to take ownership of the data, then it may be the case that string
by value is better than string_view
.
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