How to return a constant view of a std::string?

Question

While prototyping and playing around in C++, trying some concepts for making a utf8-aware immutable string, I stood with the following dilemma:

Is there any way to return a immutable view of a string. Like, instead of returning a substring, I want to be able to return a substring that references a part of the original string.

// Just some quick prototyping of ideas.
// Heavier than just a normal string.
// Construction would be heavier too because of the indices vector.
// Size would end up being O1 though.
// Indexing would also be faster.

struct ustring {
    std::string data;
    std::vector<size_t> indices;

    // How do I return a view to a string?

    std::string operator [](size_t const i) const {
        return data.substr(indices[i], indices[i + 1] - indices[i]);
    }
};

Do you have the string_view class available to you from c++17? — Paul Rooney
– Paul Rooney, Commented Jan 17, 2017 at 0:29
alternately, some libraries implemented <experimental/string_view> in c++14, and before that, boost had a string_view library. You could also use the GSL. — jaggedSpire
– jaggedSpire, Commented Jan 17, 2017 at 0:34

Travis Gockel · Accepted Answer · 2017-01-17 00:38:22Z

5

Sounds like std::string_view is the class for you! If you don't have C++17 support, then try out std::experimental::string_view. If that's not available, try out boost::string_view. All of those choices can be used in the same way (just replace std::string_view with whatever you use):

std::string_view operator [](size_t const i) const {
    return std::string_view(&data[i], 1);
}

Welcome to C++, where there's always another kitchen sink!

edited Jan 17, 2017 at 0:38

answered Jan 17, 2017 at 0:34

Travis Gockel

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7 Comments

João Pires Over a year ago

In Visual Studio 2017 the header does appear in the intellisense, however, when I do std:: there's no string_view nor experimental::string_view available. Maybe they just reserved the word for future use?

Travis Gockel Over a year ago

Did you #include <string_view> or #include <experimental/string_view>?

João Pires Over a year ago

#include <string_view>

Travis Gockel Over a year ago

Does compilation fail at the #include or at the attempt to use std::string_view? (I'm grasping a bit here -- I don't really use Visual Studio or MSVC)

Travis Gockel Over a year ago

Let us continue this discussion in chat.

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