Is there a length limit on g++ variable names?

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Short Answer:

No

Long Answer:

Yes, it has to be small enough that it will fit in memory, but otherwise no, not really. If there is a builtin limit (I don't believe there is) it is so huge you'd be really hard-pressed to reach it.

Actually, you got me really curious, so I created the following Python program to generate code:

#! /usr/bin/env python2.6
import sys;
cppcode="""
#include <iostream>
#include <cstdlib>

int main(int argc, char* argv[])
{
     int %s = 0;
     return 0;
}
"""

def longvarname(n):
    str="x";
    for i in xrange(n):
        str = str+"0";
    return str;

def printcpp(n):
    print cppcode % longvarname(n);

if __name__=="__main__":
    if len(sys.argv)==2:
        printcpp(int(sys.argv[1]));

This generates C++ code using the desired length variable name. Using the following:

./gencpp.py 1048576 > main.cpp
g++ main.cpp -o main

The above gives me no problems (the variable name is roughly 1MB in length). I tried for a gigabyte, but I'm not being so smart with the string construction, and so I decided to abort when gencpp.py took too long.

Anyway, I very much doubt that gcc pre-allocates 1MB for variable names. It is purely bounded by memory.

an additional gotcha, some linkers have a limit on the length of the mangled name. this tends to be an issue with template and nested classes more than identifier length but either could trigger a problem afaik

I don't know what the limit is (or if there is one), but I think it is good practice that there should be one, in order to catch pathological code, for example that created by a runaway code generator. For what it's worth, the C++ Standard suggests a minimum of 1K for identifier length.