In the for loop, size of vector is calculated for the maximum number of iterations of loop and using at(), the elements are printed. for(int i=0; i < a. size(); i++) std::cout << a.at(i) << ' '; In the main() function, the elements of vector are passed to print them.
How do we find an element using STL? Approach 1: Return index of the element using std::find() Use std::find_if() with std::distance() Use std::count()
The usual way to examine data in your program is with the print command (abbreviated p ), or its synonym inspect . It evaluates and prints the value of an expression of the language your program is written in (see section Using GDB with Different Languages).
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
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).
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
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