Eclipse C++ debugger not displaying variable values

Question

I have used Eclipse for Java coding without any problem. With C++ Eclipse (Indigo), my problem is that, I am not able to see the values of the variables when I put the mouse over them ! It just shows the definition of the variable.

In Java if we right click on a variable then we get an option like "Inspect value" . That option is also not visible in C++ eclipse. How to solve this issue ? Is there any plugin or configuration am I missing ?

Have freshly installed Ubuntu 11.10 in Virtual Box (Windows XP Host). Then installed g++ 4.6, Eclipse Indigo and Eclipse CDT. In "Debug Configurations" , it shows:

Debugger: gdb/mi
Advanced: Automatically track values of "Variables" and "Registers"
GDB Debugger: gdb
GDB command file: .gdbinit
GDB Command set: Standard(Linux)
Protocol: mi
(unchecked) Verbose console mode
(unchecked) Use full file path to set breakpoints

I am able to put break points and stop the execution there, the only problem is seeing values.

Answer 1

Most likely, due to optimizations those values don't exist at that point. For example, consider:

int foo(void)
{
    int i=SomeFunctions();
    if(i>3) Foo();
    else
    {
       Bar();
       Baz();
    }
    // breakpoint here
    return 8;
}

The compiler is free to keep i entirely in a register. By the time you hit the breakpoint, that register may have been re-used for some other purpose. It can be impossible for the debugger to tell you the value of i at that point.

As a general rule, unless the execution of a program depends on the value of a variable, there is no requirement that it be possible to determine that variable's value.

Turning off optimizations can help.

Answer 2

It seems possible that the compiler is optimizing it out. A quick trick to make sure a variable will never be optimized out is to declare it as volatile . This tells the compiler that the variable should be treated like it could changed at any time (such as a global being modified from an interrupt).

Example:

int main()
{
   // Even though we never read the value of test, it will not be optimized away
   volatile int test;

   test = foo();

   return 0;
}