Local Variables In Control Flow Statements

if Locals: Basis

code/local-ctlflow-if-simple.cpp

#include <iostream>

unsigned foo()
{
    return 42;
}

int main()
{
    if (unsigned f = foo(); f%2==0)
        std::cout << "foo returned " << f << " which is even\n";
    else
        std::cout << "foo returned " << f << " which is odd\n";
    // std::cout << "foo returned " << f << '\n';      // <-- f only valid in if/else
    return 0;
}

if Locals: Use Case: std::map<> Lookup

code/local-ctlflow-if-map-found.cpp

#include <map>
#include <iostream>

const std::map<int, const char*> int_words{
    { 42, "fourty-two" },
    {  7, "seven" },
};

int main()
{
    if (auto found = int_words.find(42); found != int_words.end())
        std::cout << found->second << '\n';
    else
        std::cout << "bummer\n";
    return 0;
}

if Locals: Use Case: Scoped Locking

code/local-ctlflow-if-refcount-lock.cpp

#include <mutex>
#include <iostream>

std::mutex lock;
int shared_count = 0;
const char* shared_resource;

void shared_resource_alloc()
{
    shared_resource = new const char[]{"Howdy"};
}
void shared_resource_free()
{
    delete[] shared_resource;
}

int main()
{
    if (std::scoped_lock _(lock); shared_count == 0) {
        shared_resource_alloc();
        ++shared_count;
    }

    std::cout << shared_resource << '\n';

    if (std::scoped_lock _(lock); shared_count == 1)
        shared_resource_free();

    return 0;
}

for Locals: Good Old Times

Traditional for loop has always had locals

code/locals-ctlflow-for-goodold.cpp

#include <iostream>

int main()
{
    for (int i=0; i<10; i++)                           // <-- for-local (since ages)
        std::cout << i << std::endl;
    return 0;
}

Range for Locals: Only Logical ⟶ New In C++20

code/locals-ctlflow-rangefor.cpp

#include <iostream>
#include <array>

std::array numbers = {3, 1, 2, 4, 7, 100};

int main()
{
    for (auto size=numbers.size(); int n: numbers) {
        std::cout << n;
        if (--size > 0)
            std::cout << ',';
        else
            std::cout << '\n';
    }
    return 0;
}