Expects/Ensures do not trigger breakpoints??

[Kai456]

Hi Guys,

I'm learning about pre and post conditions and how to apply them. I am told the Expect/Ensures macros from GSL are the way to go. As far as I understand these macros are supposed to trigger a breakpoint and "lead" you to the origin of the error, however instead of doing this they simply give me a debug error with no really tangible information! Am I doing something wrong here? Thanks!

#include <iostream>
#include <string>
#include <gsl/gsl>
using namespace std;

inline void empty_string_check(const string* const p) {
	Ensures(*p != "");
}

int main() {
	empty_string_check(new string(""));
	
	string x;
	getline(cin, x);
	return 0;

}

[icy1]

I'm learning about pre and post conditions and how to apply them.

okay

I am told the Expect/Ensures macros from GSL are the way to go.

er, not sure how this relates to previous statement, but okay...
Edit: never mind; precondition and postcondition (one word, not two) -- something that must always be true before or after a piece of code.

See this answer on defining GSL_THROW_ON_CONTRACT_VIOLATION before your #include (line 3) ; might give you more info depending on your OS https://stackoverflow.com/questions/36349523/ensures-guideline-support-library

The problematic line number might already be obvious, though. Hopefully that lib gives you some info about local vars at the time of the crash too.

[icy1]

found the public file for gsl_assert from Microsoft's github, which contains definitions for expects/ensures. Made a little project at https://repl.it/@icy_1/QuarterlyOldlaceHypotenuse to play around with it.

As per the comments in gsl/gsl_assert file, the default behavior is simply to terminate, but once the define is in play you get a file and line number. Then it's up to you to automatically breakpoint when uncaught C++ exceptions are thrown. In newer Visual Studio s, it's under Debug->Windows->Exception Settings, and then check off "C++ Exceptions".

btw, I think this would be considered a precondition -- checking the string for emptiness, etc, before doing something with it. For example:

#include <iostream>
#include <string>

#define GSL_THROW_ON_CONTRACT_VIOLATION 1
#include "gsl/gsl_assert"

using namespace std;

void FirstTwoCharacters(const string& s)
{
    Expects(!s.empty() && s.size()>=2);
    cout << s[0] << '\n' << s[1] << '\n';
}

int main() 
{
    string s("12");
    FirstTwoCharacters(s);

    string bad("a");
    FirstTwoCharacters(bad);

    return 0;
}

[Kai456]

Thanks icy!