Re: Infinite look ahead required by C++?

"ng2010" <ng2010@att.invalid> writes:


> What elements of C++ make it so hard to parse? Is it a weakness of
> compiler designs rather than a weakness of the language design? I've read
> somewhere that the language requires potentially infinite look ahead.
> Why? And how do compilers handle it?
> [It's ambiguous syntax. Others can doubtless fill in the details. -John]


The main ambiguity problem is very simple. Declarations and
expressions can look the same, and if the text can be an expression it
is, otherwise it is a declaration. Inside an expression and a
declaration, things are not always broken into the same fragments
(non-terminals), so you need to know which you are parsing to know how
to divide the text up. However, you may need to parse arbitrarily far
ahead to get to the text which is legal only if it is treated as a
declaration (or only legal in an expression) to disambiguate.


In the following example, I build up the issue slowly, notice how the
last two differ.


typedef class c *p;
class d {
    p x; // decl
    p(y); // decl
    p(f(p(x))); // prototype
    p(z(p(x))) { // defn
        p(z(p(x))); // expression
        p(f(p(q))); // prototype
return 0;
};
    };


Hope this helps,
-Chris


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Chris Clark email: christopher.f.clark@compiler-resources.com
Compiler Resources, Inc. Web Site: http://world.std.com/~compres
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Berlin, MA 01503 USA twitter: @intel_chris
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