gdb : setting breakpoint when new file is opened or stream instance is created - c++

Question

I have a large code and it is not obvious when it is creating an output file. Is there a way in gdb to put a breakpoint when a new ofstream is created? Or when a file is written to?
I've tried things like
(gdb) b ofstream
Function "ofstream" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) b std::ofstream
Function "std::ofstream" not defined.
Make breakpoint pending on future shared library load? (y or [n]) n
I want to do this so that I can get a backtrace to find out which functions are creating this file.
I also tried
(gdb) catch syscall write
This works except it also catches the output to the screen (and the output to stdout is verbose), when really I want to catch the output to a file.
EDIT: Here is a minimal working example.
#include <iostream>
#include <fstream>
using namespace std;
int main () {
cout << "hello world\n" ;
ofstream myfile;
myfile.open ("example.txt");
myfile << "Writing this to a file.\n";
myfile.close();
return 0;
}

Answer 1

This works except it also catches the output to the screen
You can exclude output to stdout with conditional breakpoint. On x86_64 this can be done with comparing rdi register to 1:
(gdb) catch syscall write
Catchpoint 1 (syscall 'write' [1])
(gdb) condition 1 $rdi!=1
(gdb) i b
Num Type Disp Enb Address What
1 catchpoint keep y syscall "write"
stop only if $rdi!=1
(gdb)
You can also set conditional breakpoint on exact file descriptor you want. See https://stackoverflow.com/a/8052681/72178 how to map file name to file descriptor on Linux.

Answer 2

Unfortunately you have to provide the complete function prototype. But gdb can give you a list of available methods and functions. If you type (note the single quote)
(gdb) break 'std::
you can invoke the auto-completion with tabulator. This will return a list of all gdb known methods and functions under namespace std. In your case (ofstream) you will find following entry in the list:
std::basic_ofstream<char, std::char_traits<char> >::basic_ofstream(char const*, std::_Ios_Openmode)```
Now just set the breakpoint with following command:
(gdb) break std::basic_ofstream<char, std::char_traits<char> >::basic_ofstream(char const*, std::_Ios_Openmode)
run it and it should work as expected.