unable to include multiple expressions in initialization of a variable c++

Question

It is stated on the site cppreference.com, something like that

For each declarator, the initializer may be one of the following:

( expression-list ) (1)

= expression (2)

{ initializer-list } (3)

1. comma-separated list of arbitrary expressions and braced-init-lists in parentheses

But in my code

int main(){

    int a,b=5,c(a,b);
    return 0;
}

when I try to compile, the following error occurs

...error: expression list treated as compound expression in initializer [-fpermissive]

My question is, if list of multiple expressions is allowed in such style of initialization, then why the compiler is not accepting it with variable c? What am I missing?

Answer 1

All right, let's look at this:

int main(){

    int a,b=5,c(a,b);
    return 0;
}

What do you expect c(a,b) to actually do?

Let's simplify this just slightly:

int main(){

    int a,b=5;
    int c(a,b);
    return 0;
}

This will generate the same syntax error, but it now stands alone. So...

Your code would work if there were a constructor for int that took two ints as parameters. This would also compile:

int c(int a, int b);

But in that case, you're actually defining a function.

Also, this works:

int main() {
    int a = 5;
    int b = 10;
    int c(b);

    std::cout << "C == " << c << std::endl;
}

That works because an int can be initialized from a single int. But you're getting an error because you can't initialize an int from two other ints.

This works:

#include <iostream>

class MyClass {
public:
    MyClass(int a, int b): value(a + b) {}
    int value;
};

int main() {
    int a = 5;
    int b = 10;
    MyClass c(a, b);

    std::cout << "C == " << c.value << std::endl;
}

And maybe that's what the article you read was trying to tell you. Note: cpppreference is NOT a good site for learning C++. Get a good book.