C++ Can I write a function within an expression?

Question

I'm writing a (my own language) to C++ transpiler. I'd like to be able to offer a kind of virtual sub function that is transpiled into being part of the C++ code in the same scope in which it is called.

Here is an example of the type of C++ code I'd want to generate:

if (a == {if (getValue(d) == 5) { long r = next(); next(); return r; } else return 0;})
dosomething();

It says if a is equal to the result of what's in the brackets, then dosomething().

You can see the idea is to inline a function within an expression. Yes that could be done by allowing the compiler to inline, but in this case I want to keep access to the variables in the outer scope, which I would lose if it actually were another function.

Any decent way to do this?

@HolyBlackCat isn't a lambda only one statement? In my inlined function it could be a hundred lines long with all the bells and whistles of normal code. — Alasdair, Apr 12 '21 at 14:27
Not exactly sure I get the question, but isn't lambdas what you want? You can define and immidiatly execute lambda inside IF statement. — Dmitry, Apr 12 '21 at 14:28
@HolyBlackCat, ah see this is the problem of thinking you know a little is worse than knowing nothing at all :) Thanks! — Alasdair, Apr 12 '21 at 14:30

bartop · Accepted Answer · 2021-04-12T14:50:37.540

4

If I correctly get what you are doing, you can simply use lambda expression here. This of course requires C++11

if (a ==
   [&] () {
        if (getValue(d) == 5) { 
            long r = next();
            next(); 
            return r;
        } else { 
            return 0;
        } 
    }()) {
    dosomething();
}

If you are writing translator/transpiler you may want to try more explicit capture list than this generic [&].

edited Apr 12 '21 at 14:50

answered Apr 12 '21 at 14:29

bartop

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@Galik thanks, fixed it – bartop Apr 12 '21 at 14:50

C++ Can I write a function within an expression?

1 Answers1