Segmentation Fault in Constructor

Question

This happens with EVERY class I try to make in C++. Migrating from java, I find problems mainly in making classes. I run valgrind and it's in the constructor, it appears to be.

==30214== Memcheck, a memory error detector
==30214== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==30214== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==30214== Command: ./CoC
==30214== 
==30214== 
==30214== Process terminating with default action of signal 11 (SIGSEGV)
==30214==  Bad permissions for mapped region at address 0x404B4F
==30214==    at 0x4C2B9EC: strcat (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==30214==    by 0x404220: Model::Model(std::string) (in /home/kronus/Apollo/CoC)
==30214==    by 0x402617: main (in /home/kronus/Apollo/CoC)

As you can see I'm trying to call the constructor of this model class into the main method. Here's the code for the constructor

Model::Model(std::string filename)
{
m_TotalFaces = 0;
m_model = lib3ds_file_load(filename.c_str());
    // If loading the model failed, we throw an exception
    if(!m_model)
    {
           throw strcat("Unable to load ", filename.c_str());
    }
}

When it's called it closes with a segmentation fault. Important: I have declared the class inside the header file. This is when I get the error. I put the class inside the source file and it runs fine. What am I doing wrong?

Answer 1

strcat attempts to write the string pointed to by the second argument at the end of the string pointed to by the first argument. Since the first argument is a string literal (which should be considered read only), you get a nasty segfault.

I suggest learning C++ as though it's an entirely different language to Java, because otherwise you may think that similar features function the same. That's dangerous. A monkey can learn Java by mashing it's face on the keyboard. C++ has undefined behaviour which may appear to function correctly on your machine, but launch nuclear missiles on another.

Answer 2

You are appending to a constant string that is wrong:

strcat("Unable to load ", filename.c_str());
         ^ you can't append to constant

Read this: c++ exception : throwing std::string
You may want to avoid using strings as exception classes because they themselves can throw an exception during use.

second: What type should I catch if I throw a string literal?