| Thinking in C++ Vol 2 - Practical Programming |
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Sometimes it s useful to print the code of each statement as
it is executed, either to cout or to a trace file. Here s a preprocessor
macro to accomplish this:
#define TRACE(ARG) cout << #ARG << endl; ARG
Now you can go through and surround the statements you trace
with this macro. However, this can introduce problems. For example, if you take
the statement:
for(int i = 0; i < 100; i++)
cout << i << endl;
and put both lines inside TRACE( ) macros, you
get this:
TRACE(for(int i = 0; i < 100; i++))
TRACE( cout << i << endl;)
which expands to this:
cout << "for(int i = 0; i < 100;
i++)" << endl;
for(int i = 0; i < 100; i++)
cout << "cout << i <<
endl;" << endl;
cout << i << endl;
which isn t exactly what you want. Thus, you must use this
technique carefully.
The following is a variation on the TRACE( )
macro:
#define D(a) cout << #a "=[" << a <<
"]" << endl;
If you want to display an expression, you simply put it
inside a call to D( ). The expression is displayed, followed by its
value (assuming there s an overloaded operator << for the result
type). For example, you can say D(a + b). You can use this macro any
time you want to check an intermediate value.
These two macros represent the two most fundamental things
you do with a debugger: trace through the code execution and display values. A
good debugger is an excellent productivity tool, but sometimes debuggers are
not available, or it s not convenient to use them. These techniques always
work, regardless of the situation.
| Thinking in C++ Vol 2 - Practical Programming |
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