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Structured binding

类比Python assignment

在Comparison-of-programming-language\Cpp-VS-Python中对两者进行了类比，只能够帮助我们理解这个特性。

codeproject C++ 17 New Features and Trick # Structure Binding

Multiple return values from functions are not a new concept in programming and similar functionality is present in many other programming languages. C++17 comes with a new feature (structured bindings) that provides functionality similar to the multiple return values provided in other languages.

In the following example, I want to provide an overview of some of the options that we have in the old C++ standard, in the modern standard (C++11/14) and today in C++17 to return multiple values from functions:

#include <iostream>
#include <tuple> // std::tie
#include <math.h>       /* sin */

const double PI = 3.14159265;

void calculateSinCos(const double param, double &resSin, double &resCos)
{
    resSin = sin(param * PI / 180.0); // converting degrees to radians
    resCos = cos(param * PI / 180.0); // converting degrees to radians
}

std::pair<double, double> calculateSinCos(const double param)
{
    return
    {   sin(param * PI / 180.0), cos(param * PI / 180.0)};
}

std::tuple<double, double> calculateSinCos_Tuple(const double param)
{
    return std::make_tuple(sin(param * PI / 180.0), cos(param * PI / 180.0));  // packing values into tuple
}

int main()
{
    double param { 90.0 };
    double resultSin { 0.0 };
    double resultCos { 0.0 };

    //C++98
    calculateSinCos(param, resultSin, resultCos);
    std::cout << "C++98 : sin(" << param << ") = " << resultSin << ", cos(" << param << ") = " << resultCos << "\n";

    //C++11
    const auto resSinCos(calculateSinCos(param));
    std::cout << "C++11 : sin(" << param << ") = " << resSinCos.first << ", cos(" << param << ") = " << resSinCos.second << "\n";

    //C++11
    std::tie(resultSin, resultCos) = calculateSinCos(param);
    std::cout << "C++11 : sin(" << param << ") = " << resultSin << ", cos(" << param << ") = " << resultCos << "\n";

    // C++17
    auto [a, b] = calculateSinCos(param);
    std::cout << "C++17 :sin(" << param << ") = " << a << ", cos(" << param << ") = " << b << "\n";

    // C++17
    auto [x, y] = calculateSinCos_Tuple(param);
    // std::make_tuple(sin(val), cos(val));
    std::cout << "C++17 : sin(" << param << ") = " << x << ", cos(" << param << ") = " << y << "\n";
}

Alternatively, before C++17, it would be possible to use std::tie to unpack a tuple/pair to achieve a similar effect:

std::tie(resultSin, resultCos) = calculateSinCos(param);

This approach demonstrates how to unpack the resulting pair into two variables. Notice, this example shows all the power of std::tie, but nonetheless, the std::tie is less powerful than structured bindings, because we must first define all the variables we want to bind.

Structured binding is a new functionality of C++17, making the code even more readable, expressive and concise.

auto[a, b] = calculateSinCos(param);

Notice, the variables a and b are not references; they are aliases (or bindings) to the generated object member variables. The compiler assigns a unique name to the temporary object.

In C++11, the std::tuple container has been added to build a tuple that contains multiple return values. But neither C++11 nor C++14 does support an easy way to get elements in a std::tuple directly from the tuple (Of course, we can unpack a tuple using std::tie, but we still need to understand the type of each object and how many objects are this tuple. Phew, how painful it is...)

C++17 fixes this flaw, and the structured bindings allow us to write code as follows:

auto[x, y] = calculateSinCos_Tuple(param);

geeksforgeeks Structured binding in C++

#include <bits/stdc++.h> 
#include <map> 
using namespace std; 

int main() 
{ 
    // Creating a map with key and value  
    // fields as String 
    map<string, string> sites; 

    sites.insert({ "GeeksforGeeks", "Coding Resources" }); 
    sites.insert({ "StackOverflow", "Q-A type" }); 
    sites.insert({ "Wikipedia", "Resources + References" }); 

    for (auto & [ key, value ] : sites)  
    { 
       cout << key.c_str() << " " << value.c_str() << endl; 
    } 

    return 0; 
}

cppreference Structured binding declaration (since C++17)

思考

1) Python a, b = b, a swaps two variables，那C++中呢？

2) 下面程序在C++中的运行结果如何呢？

x = [0, 1]
i = 0
i, x[i] = 1, 2         # i is updated, then x[i] is updated
print(x)

Python中的运行结果如下:

[0, 2]