gdb: how do you get the max value of a std::vector<float>?

Question

I am debugging some C++ code and I have a really large std::vector<floating_point_type> (float or double). In C++ code when I want the max value of the std::vector I can just use std::max_element(). So in gdb I tried to use the same code, but I got an error.

In gdb I used call std::max_element(x) and got an error message No symbol "max_element" in namespace "std".

Is there any way to get the max value of a vector in gdb?

I would also appreciate an explanation for why my attempt at using std::max_element was not working (perhaps std::max_element a header-only or inline function).

Answer 1

std::max_element is a function template, not a function. You are asking GDB to do template argument deduction and the whole shebang involved with calling a template function without specifying arguments. It can't do that, naturally, it's not a full fledged compiler.

As far as I know any solution, from the simplest to the most complex, will require of you to modify your source in such a way that std::max_element is instantiated for the iterator types of your vector. So you may as well add "debug only" code that computes the maximum element and stores it into a local variable.

Answer 2

(gdb) p std::max⭾⭾⭾

(No response from gdb)

 (gdb) p std::max_element(v.begin(), v.end())
 No symbol "max_element" in namespace "std".

Here is how to call an uninstantiated function template from gdb without breaking your session and recompiling your program.

1. Open an editor and create a C++ source file with an explicit instantiation of the needed function. For example:

   // /tmp/tmpsource.cpp
   #include <algorithm>
   #include <vector>
   using T = std::vector<int>::iterator;
   template T std::max_element<T>(T, T);
   

   This can probably be automated somehow with a shell script. Give it a name of a function and template arguments, and it will generate you a complete buildable C++ source. Probably not worth the trouble though.

2. Build a shared library/DLL from the source.

   g++ -fPIC -shared -ggdb -O0 -o /tmp/libtmpsource.so /tmp/tmpsource.cpp
   

3. Load the library in your debugging session.

   load /tmp/libtmpsource.so
   # if this doesn't work
   p dlopen("/tmp/libtmpsource.so", 2)
   # or perhaps even
   p LoadLibraryA("c:/temp/libtmpsource.so")
   

4. Let's try the function now.

   (gdb) p std::max_element(v.begin(), v.end())
   No symbol "max_element" in namespace "std".
   

5. What? No worries, everything is under control. gdb is not a C++ compiler and it cannot do the template deduction thing. You need to specify your <...> yourself. Fortunately, autocompletion works (sometimes).

   (gdb) p std::max⭾
   (gdb) p std::max_element<__gnu_cxx::__normal_iterator<int*, std::vector<int, 
    std::allocator<int> > > >(__gnu_cxx::__normal_iterator<int*, std::vector<int, 
    std::allocator<int> > >, __gnu_cxx::__normal_iterator<int*, std::vector<int, 
    std::allocator<int> > >)