我什么时候会通过 const&std::string 而不是 std::string_view?

2022-01-07 00:00:00 string parameter-passing c++ stl string-view

我了解使用 std::string_view 的动机；
它可以帮助避免函数参数中不必要的分配.

I understand the motivation for using std::string_view;
it can help avoid unecessary allocations in function arguments.

例如:
以下程序将从字符串文字创建 std::string.
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]
";
    return malloc(n);
}

void observe_string(std::string const& str){}

int main(){
  observe_string("hello world"); //prints [allocating 36 bytes]
}

使用 string_view 可以解决问题:

Using string_view will solve the problem: 

#include <iostream>
#include <experimental/string_view>

void* operator new(std::size_t n)
{
    std::cout << "[allocating " << n << " bytes]
";
    return malloc(n);
}

void observe_string(std::experimental::string_view const& str){
}

int main(){
  observe_string("hello world"); //prints nothing
}

<小时>

这给我留下了一个问题.
我什么时候会通过 const& 选择 std::string而不是 string_view 函数参数?

看std::string_view的接口，看起来好像可以替换const&传递的std::string的所有实例..有什么反例吗?std::string_view 是否意味着替换 std::string const& 用于参数传递?

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

我什么时候会通过 const& 选择 std::string 而不是 string_view 作为函数参数?

When would I choose std::string by const& instead of string_view for function arguments?

您需要一个以空字符结尾的字符串吗?如果是这样，那么您应该使用 std::string const& 来保证.string_view 不是 - 它只是一个 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.

如果您不需要一个以空字符结尾的字符串，并且您不需要取得数据的所有权，那么您应该使用string_view.如果您确实需要获得数据的所有权，那么 string 按值可能比 string_view 更好.

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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