How do I print the elements of a C++ vector in GDB?
I want to examine the contents of a std::vector in GDB, how do I do it? Let's say it's a std::vector<int> for the sake of simplicity.
To view vector std::vector myVector contents, just type in GDB:
(gdb) print myVector
This will produce an output similar to:
$1 = std::vector of length 3, capacity 4 = {10, 20, 30}
To achieve above, you need to have gdb 7 (I tested it on gdb 7.01) and some python pretty-printer. Installation process of these is described on gdb wiki.
What is more, after installing above, this works well with Eclipse C++ debugger GUI (and any other IDE using GDB, as I think).
With GCC 4.1.2, to print the whole of a std::vector<int> called myVector, do the following:
print *(myVector._M_impl._M_start)@myVector.size()
To print only the first N elements, do:
print *(myVector._M_impl._M_start)@N
Explanation
This is probably heavily dependent on your compiler version, but for GCC 4.1.2, the pointer to the internal array is:
myVector._M_impl._M_start
And the GDB command to print N elements of an array starting at pointer P is:
print P@N
Or, in a short form (for a standard .gdbinit):
p P@N
'Watching' STL containers while debugging is somewhat of a problem. Here are 3 different solutions I have used in the past, none of them is perfect.
1) Use GDB scripts from http://clith.com/gdb_stl_utils/ These scripts allow you to print the contents of almost all STL containers. The problem is that this does not work for nested containers like a stack of sets.
2) Visual Studio 2005 has fantastic support for watching STL containers. This works for nested containers but this is for their implementation for STL only and does not work if you are putting a STL container in a Boost container.
3) Write your own 'print' function (or method) for the specific item you want to print while debugging and use 'call' while in GDB to print the item. Note that if your print function is not being called anywhere in the code g++ will do dead code elimination and the 'print' function will not be found by GDB (you will get a message saying that the function is inlined). So compile with -fkeep-inline-functions
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