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Usually functions return single-valued results: tt(doubles, ints, strings),
etc. When functions need to return multiple values a em(return by argument)
construction is often used, where addresses of variables that live outside of
the called function are passed to functions, allowing the functions to assign
new values to those variables.
When multiple values should be em(returned) from a function a tt(struct) can
be used, but em(pairs) (cf. section ref(PAIR)) or em(tuples) (cf. section
ref(TUPLES)) can also be used. Here's a simple example, where a function
tt(fun) returns a tt(struct) having two data fields:
verb( struct Return
{
int first;
double second;
};
Return fun()
{
return Return{ 1, 12.5 };
})
(Briefly forward referencing to sections ref(PAIR) and ref(TUPLES): the
tt(struct) definition can completely be omitted if tt(fun) returns a
tt(pair) or tt(tuple). In those cases the following code remains valid.)
A function em(calling) tt(fun) traditionally defines a variable
of the same type as tt(fun's) return type, and then uses that variable's
fields to access tt(first) and tt(second). If you don't like the typing,
tt(auto) can also be used:
verb( int main()
{
auto r1 = fun();
cout << r1.first;
})
Instead of referring to the elements of the returned tt(struct, pair) or
tt(tuple) em(structured binding declarations) can also be used. Here, tt(auto)
is followed by a (square brackets surrounded) comma-separated list of
variables, where each variable is em(defined), and receives the value of the
corresponding field or element of the called function's return value. So, the
above tt(main) function can also be written like this:
verb( int main()
{
auto [one, two] = fun();
cout << one; // one and two: now defined
})
Merely specifying tt(auto) results in tt(fun's) return value being
copied, and the structured bindings variables will refer to the copied value.
But structured binding declarations can also be used in combination with
(lvalue/rvalue) return values. The following ensures that tt(rone) and
tt(rtwo) refer to the elements of tt(fun's) anonymous return value:
verb( int main()
{
auto &&[rone, rtwo] = fun();
})
If the called function returns a value that survives the function call
itself, then structured binding declarations can use em(lvalue
references). E.g.,
verb( Return &fun2()
{
static Return ret{ 4, 5 };
return ret;
}
int main()
{
auto &[lone, ltwo] = fun2(); // OK: referring to ret's fields
})
To use structured binding declarations it is not required to use function
calls. The object providing the data can also anonymously be defined:
verb( int main()
{
auto const &[lone, ltwo] = Return{ 4, 5 };
// or:
auto &&[lone, ltwo] = Return{ 4, 5 };
})
The object doesn't even have to make its data members publicly
available. In section tt(TUPLES) using structured bindings not necessarily
referring to data members is covered.
Another application is found in situations where nested statements of
tt(for) or selection statements benefit from using locally defined variables
of various types. Such variables can easily be defined using structured
binding declarations that are initialized from anonymous structs, pairs or
tuples. Here is an example illustrating this:
verb( // define a struct:
struct Three
{
size_t year;
double firstAmount;
double interest;
};
// define an array of Three objects, and process each in turn:
Three array[10];
fill(array); // not implemented here
for (auto &[year, amount, interest]: array)
cout << "Year " << year << ": amount = " << amount << '\n';)
When using structured bindings the structured binding declaration must
specify all elements that are available. So if a struct has four data members
the structured binding declaration must define four elements. To avoid
warnings of unused variables at lease one of the variables of the structured
binding declaration must be used.
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