CODEWITHSUNDEEP
c++c++17
Alex
Alex

Reputation: 10126

Initialization in if statement existed before C++17?

Accidentally the following code was found:

if (bool result = f()) {
    // Do the stuff
}

It was compiled with gcc 4.9.2 and MSVS 2013.

The following code compiles and prints False!:

#include <iostream>

bool foo() {
    return false;
}

void bar() {
    if (bool result = foo()) {
        std::cout << "True!\n";
    } else {
        std::cout << "False!\n";
    }
}

int main()
{
    bar();

    return 0;
}

I thought that this (except syntax) functionality is available only since C++17.

Do I understand it wrong?

Upvotes: 3

Views: 256

Answers (4)

Bhupesh Raj
Bhupesh Raj

Reputation: 31

Yes. In c++17, we can initialize and then test a condition in if statement.

Example : 

 if( bool result = foo(); result ) {
    std::cout << "True!\n";
 } else {
    std::cout << "False!\n";
 }

It helps us write a cleaner code, if the scope of the variable is just limited to the if statement.

The above will look neat than

 bool result = foo();
 if( result ) {
    std::cout << "True!\n";
 } else {
    std::cout << "False!\n";
 }

Upvotes: 1

Richard Hodges
Richard Hodges

Reputation: 69892

c++17 also allows this potentially confusing construct:

int bar();
bool query();
void foo(int, bool);

int main()
{
    if (int x = bar() ; bool y = query())
    {
        foo(x, y);
    }
    else
    {
        foo(x * 2, y);
    }
}

Upvotes: 1

Matthieu Brucher
Matthieu Brucher

Reputation: 22023

The syntax in c++17 is different:

if(int foo = result(); foo == 1)

The new notation first declares the variable and then makes a test on it. It fixes the issue of assignment and condition test in the same statement that could lead to mistakes.

Upvotes: 4

Rakete1111
Rakete1111

Reputation: 48958

Do I understand it wrong?

Jup. It was always allowed to have a declaration within the condition of an if statement. What is new in C++17 is that you can have a initializer and a condition:

if (int A = 0; ++A == 1);
//  ^^^^^^^^^
//  new part

For those asking why this is a useful feature, here's an example that I like from Reddit:

std::map<int, std::string> Map;
// ...

if (auto[it, inserted] = Map.insert(std::pair(10, "10")); inserted)
    ; // do something with *it.

Upvotes: 14

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