Why does the debugger always skip one line in this program? [duplicate]

Question

I am writing a template class of a matrix named Matrix, and I rewrite the default constructor like this:

template<typename _Tp, size_t m, size_t n> inline Matrix<_Tp, m, n>::Matrix() { for(size_t i = 0; i != m*n; ++i) val[i] = _Tp(0); }

And in my test file I write this:

SC::Matrix<double, 3, 3> Mat();

All these are good when I build the program. But I always get wrong result when I run the test program.

And When I try to find reasons I find that the debugger always skip over definition of Mat; At the first I think that it may because I modified the files after I build this program, so I delete all the build results(automatically generated by cmake) and rebuild it. But it's useless, the problem is still there.

Is there anyone can help me find the reason? Did I provide enough information for this problem?

Is that a definition of a variable Mat that is supposed to be default initialized, or is it a declaration of a function which takes no argument and returns a SC::Matrix<double, 3, 3> object? — Some programmer dude
– Some programmer dude, Commented Aug 25, 2017 at 7:53
When you construct objects on the stack without ctor-params you don't need the (): SC::Matrix<double, 3, 3> Mat;. Otherwise it may cause your compiler to think of it as a function declaration. — Al.G.
– Al.G., Commented Aug 25, 2017 at 7:54

WindyFields · Accepted Answer · 2017-08-25 08:49:10Z

You say "...the debugger always skip over...", so I assume, you tried to create a variable Mat of type SC::Matrix<double, 3, 3> and see how it is default initialized.

If that is true, than

SC::Matrix<double, 3, 3> Mat();

declares a function called Mat taking no args and returning SC::Matrix<double, 3, 3>. And of course you can not "debug" a function declaration. If you want to create a default initialized variable write:

SC::Matrix<double, 3, 3> Mat{};

or just

SC::Matrix<double, 3, 3> Mat;

It is actually function declaration, not definition. You can debug a function definition (this is what you mostly debug, actually), but not declaration.

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