How to find where a variable takes value? gdb - watch- is time consuming, ineffective

[alwaystudent]

How to find where a variable takes value (preferably c++ source and grep,awk&sed and regex tools, variable is Integer,char* and string can get value by =, or arbitrary function without =)

-We're dealing with a great deal of code and watch- in debugger- can be done but in many cases it is time consuming and ineffective.
for direct assignments we can write:

$grep -nE '^\s*i\s*=' directory_of_code | grep -v '=='