When would I pass const& std::string instead of std::string_view?

问题

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

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]
}

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]\n";
    return malloc(n);
}

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

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

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This leaves me with a question.
When would I choose std::string by const& instead of string_view for function arguments?

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?

回答1:

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

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.

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.

回答2:

One possible reason to accept const std::string& instead of string_view is when you want to store reference to string object which can change later.

If you accept and store a string_view, it might become invalid when string internal buffer reallocates.

If you accept and store reference to string itself, you won't have that problem, as long as that object is alive (you probably want to delete r-value reference overload, to avoid obvious problem with temporaries).

回答3:

Andrei Alexandrescu once said, "No Work is better than some work". So you should use const std::string& in such contexts. Because std::string_view still involves some work (copying a pair of pointer and length).

Of course, const references may still have the cost of copying a pointer; which is almost the equivalent of what std::string_view will do. But there's one additional work with std::string_view, it also copies the length.

This is in theory, but in practice, a benchmark will be preferred to infer performance

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回答4:

It is not really what you were asking, but sometimes you want to take std::string by value rather than std::string_view for performance reasons. This is the case when you will need to modify the string before inspecting it:

bool matches(std::string s)
{
  make_upper_case(s);
  return lib::test_if_matches(s);
}

You need a mutable string somewhere anyway, so you may declare it as function parameter. If you changed it to to std::string_view, and somebody passes an std::string to function matches() you would be first converting string to string_view and then string_view to string, and therefore allocating twice.

来源：https://stackoverflow.com/questions/39564457/when-would-i-pass-const-stdstring-instead-of-stdstring-view