How do I print the elements of a C++ vector in GDB?

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

Put the following in ~/.gdbinit:

define print_vector
    if $argc == 2
        set $elem = $arg0.size()
        if $arg1 >= $arg0.size()
            printf "Error, %s.size() = %d, printing last element:\n", "$arg0", $arg0.size()
            set $elem = $arg1 -1
        end
        print *($arg0._M_impl._M_start + $elem)@1
    else
        print *($arg0._M_impl._M_start)@$arg0.size()
    end
end

document print_vector
Display vector contents
Usage: print_vector VECTOR_NAME INDEX
VECTOR_NAME is the name of the vector
INDEX is an optional argument specifying the element to display
end

After restarting gdb (or sourcing ~/.gdbinit), show the associated help like this:

gdb) help print_vector
Display vector contents
Usage: print_vector VECTOR_NAME INDEX
VECTOR_NAME is the name of the vector
INDEX is an optional argument specifying the element to display

Example usage:

(gdb) print_vector videoconfig_.entries 0
$32 = {{subChannelId = 177 '\261', sourceId = 0 '\000', hasH264PayloadInfo = false, bitrate = 0,     payloadType = 68 'D', maxFs = 0, maxMbps = 0, maxFps = 134, encoder = 0 '\000', temporalLayers = 0 '\000'}}

'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

I have been able to use:

p/x *(&vec[2])@4

to print 4 elements (as hex) from vec starting at vec[2].

This is an add-up to the answer by Michał Oniszczuk.

According to this post in the issue, if you are using VSCode for development and your OS is Ubuntu 22.04, you can simply add the following code in your launch.json file.

"setupCommands": [
    {
        "description": "Test",
        "text": "python import sys;sys.path.insert(0, '/usr/share/gcc/python');from libstdcxx.v6.printers import register_libstdcxx_printers;register_libstdcxx_printers(None)",
        "ignoreFailures": false
    },
    {
        "description": "Enable pretty-printing for gdb",
        "text": "-enable-pretty-printing",
        "ignoreFailures": true
    }
]

This method simplified the answer mentioned in the gdb wiki above, since you don't have to manually specify the Python module or put this script in the .gdbinit file. And I believe this also answers the question raised by @Enlico in his comment.

By the way, if you are using Python C++ debugger in the VSCode like I do, you can use the following launch.json file out-of-the-box.

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python C++ Debug",
            "type": "pythoncpp",
            "request": "launch",
            "pythonConfig": "default",
            // "cppConfig": "default (gdb) Attach",
            "cppAttachName": "(gdb) Attach",
            "preLaunchTask": "Build"  // Specify your own build task here.
        },
        {
            "name": "(gdb) Attach",
            "type": "cppdbg",
            "request": "attach",
            "processId": "",
            "setupCommands": [
                {
                    "description": "Test",
                    "text": "python import sys;sys.path.insert(0, '/usr/share/gcc/python');from libstdcxx.v6.printers import register_libstdcxx_printers;register_libstdcxx_printers(None)",
                    "ignoreFailures": false
                },
                {
                    "description": "Enable pretty-printing for gdb",
                    "text": "-enable-pretty-printing",
                    "ignoreFailures": true
                }
            ]
        }
    ]
}

You might get a warning indicating "Missing Property 'Program'", but it seems everything works fine.

The accepted answer (Michał's) should work with recent versions of gdb. But a "full-featured" gdb is required.

On Debian 12, two packages gdb (full-featured) and gdb-minimal provide command gdb, version 13:

• gdb from package gdb could print content of std::vector.
• gdb from package gdb-minimal could not directly print the content of std::vector.