7 Features of C++17 that will simplify your code

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Structured Bindings

Do you often work with tuples?

If not, then you probably should start looking at it. Not only are tuples suggested for returning multiple values from a function, but they've also got special language support - so that the code is even easier and cleaner.

For example (got it from std::tie at cppreference):

std::set<S> mySet;
 
S value{42, "Test", 3.14};
std::set<S>::iterator iter;
bool inserted;
 
// unpacks the return val of insert into iter and inserted
std::tie(iter, inserted) = mySet.insert(value);

if (inserted)
    std::cout << "Value was inserted\n";

Notice that you need to declare iter and inserted first. Then you can use std::tie to make the magic... Still, it's a bit of code.

With C++17:

std::set<S> mySet;
 
S value{42, "Test", 3.14};

auto [iter, inserted] = mySet.insert(value);

One line instead of three! It's also easier to read and safer, isn't it?

Here's the full working code:

std::tie example

#include <cstdlib>
#include <iostream>
#include <set>
#include <string>
#include <iterator>
#include <tuple>
// { 
int main()
{
    std::set<S> mySet;
 
    // pre C++17:
    {
        S value{42, "Test", 3.14};
        std::set<S>::iterator iter;
        bool inserted;
 
        // unpacks the return val of insert into iter and inserted
        std::tie(iter, inserted) = mySet.insert(value);
        if (inserted)
            std::cout << "Value was inserted\n";
    }
    
    // with C++17:
    {
        S value{100, "abc", 100.0};
        const auto [iter, inserted] = mySet.insert(value);
        
        if (inserted)
            std::cout << "Value(" << iter->n << ", " << iter->s << ", ...) was inserted" << "\n";
    }
        
}

Also, you can now use const and write const auto [iter, inserted] and be const correct.

Structured Binding is not only limited to tuples, we have three cases:

1. If initializer is an array:

// works with arrays:
double myArray[3] = { 1.0, 2.0, 3.0 };  
auto [a, b, c] = myArray;

2. if initializer supports std::tuple_size<> and provides get<N>() function (the most common case I think):

auto [a, b] = myPair; // binds myPair.first/second

In other words, you can provide support for your classes, assuming you add get<N> interface implementation.

3. if initializer's type contains only non static, public members:

struct S { int x1 : 2; volatile double y1; };
S f();
const auto [ x, y ] = f();

Now it's also quite easy to get a reference to a tuple member:

auto& [ refA, refB, refC, refD ] = myTuple;

And one of the coolest usage (support to for loops!):

std::map myMap;    
for (const auto & [k,v] : myMap) 
{  
    // k - key
    // v - value
}

Can you help me with the code below and make it working?

iteration over map, finish the code

#include <iostream>
#include <map>
#include <string>
#include <tuple>
int main()
{
    const std::map<std::string, std::string> capitals {
        { "Poland", "Warsaw"},
        { "USA", "Washington"},
        { "France", "Paris"},
        { "UK", "London"},
        { "Germany", "Berlin"}
    };
    // can you write code that shows prints Country: Capital ?
    // eg "Poland : Warsaw" ?
    for...
}

BTW: Structured Bindings or Decomposition Declaration?

For this feature, you might have seen another name "decomposition declaration" in use. As I see this, those two names were considered, but now the standard (the draft) sticks with "Structured Bindings."

More Details in:

Section: 11.5 Structured binding declarations [dcl.struct.bind]
P0217R3
P0144R0
P0615R0: Renaming for structured bindings
c++ today: Structured Binding (C++17 inside)
C++17 Structured Bindings – Steve Lorimer

Working in GCC: 7.0, Clang: 4.0, MSVC: in VS 2017.3.

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