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?
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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 by const& instead of string_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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我什么时候可以通过const& std :: string而不是std :: string
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