I was reading Chapter 9 of the 2nd edition of A Tour of C by Bjarne Stroustrup and was puzzled by the use of {&king[0],2} where king is a string variable. I get that it returns the first 2 strings but I don't know what the name is for this to look up more details about it like:

• Does the & ampersand indicate that it is a reference or does it switch it to pointer?
• In what version was this feature introduced? I know the book is based on C 17 but don't know what version began supporting it.

// Strings and Regular Expressions
#pragma once
#include <string>


using namespace std;

string cat(string_view sv1, string_view sv2) {
    string res(sv1.length() sv2.length(), '\0');
    char* p = &res[0];

    for (char c : sv1)                  // one way
        *p   = c;
    copy(sv2.begin(), sv2.end(), p);    // another way
    return res;
}

void tryCat() {
    string king = "Harold";
    auto s1 = cat(king, "William");             // string and const char*
    auto s2 = cat(king, king);                  // string and string
    auto s3 = cat("Edward", "Stephen"sv);       // const char* and string_view
    auto s4 = cat("Canute"sv, king); 
    auto s5 = cat({&king[0],2}, "Henry"sv);     // HaHenry
    auto s6 = cat({&king[0],2}, {&king[2],4});  // Harold     
    cout << s6 << endl;
}

int main () {
    tryCat();
}

CodePudding user response：

The funcction

string cat(string_view sv1, string_view sv2) {

accepts as its arguments two objects of the type std::string_view.

Such a construction

{&king[0],2}

is used to construct an object of the type std::string_view using the constructor

constexpr basic_string_view(const charT* str, size_type len);

The expression &king[0] in the above construction has the pointer type char * and yields the address of the first character of a string.

As the variable king is declared like

string king = "Harold";

then in fact the object of the type std::string_view is built from first two characters of the string like "Ha".

CodePudding user response：

I think it's called copy-list-initialization, specifically form #7 at List initialization - Cppreference. Copy-list-initialization was introduced in C 11.

function( { arg1, arg2, ... } )           (7)

&king[0] gets the underlying char* array from the std::string. &king[2] gets the address of the third character of that array. In this case, & is the address-of operator, not a reference. [0] is a subscript. The second item in the braces after the comma is the second argument to the string_view constructor which specifies the length of the string.

Here's the description of copy-list-initialization from the cppreference link.

copy-list-initialization (both explicit and non-explicit constructors are considered, but only non-explicit constructors may be called)

6. initialization of a named variable with a braced-init-list after an equals sign
7. in a function call expression, with braced-init-list used as an argument and list-initialization initializes the function parameter