Please look at the below program and output.
using namespace std;
void mesh::openfile(std::string szFile)
if(!m_File) std::cout<< "couldn't open "<<szFile;
couldn't open input.in
TEMPLATE = app
CONFIG += console c++11
CONFIG -= app_bundle
CONFIG -= qt
SOURCES += main.cpp\
HEADERS += \
DISTFILES += \
This tries to find the file relative to the "current working directory" of the application process. By default Qt Creator sets the working directory of the application process to the directory where the application is created (or on Windows it can be one level above).
Have a look at the working directory in Projects mode > Run settings for your project, and make sure it points to the place where the input file is located. See also the Qt Creator documentation