String Mysteriously disappears bringing string from C++ to Obj C++

StackOverflow https://stackoverflow.com/questions/20752918

Question

I'm sure it's something simple, but I can't seem to figure it out. I am basically trying to use c++ to produce an HTML string and to return it to be stored in an objective c model class to be used later in a web view.

The c++ side of it seems to run fine. here's the method that's called in a cpp file:

const char *SqliteConnection::htmlTest(){
    HtmlItemValueTableBuilder htmlBuilder = HtmlItemValueTableBuilder();
    std::string s = htmlBuilder.test();
    cout << "TEST RESULT: " <<s;
    return s.c_str();
}

The cout produces the following output (which is what I want):

TEST RESULT: <!doctype html><html><head></head><div><STYLE type="text/css"> body {      margin: 10%;  }  table {    border-spacing: 5px;  }  td {     background-color: #EFEFEF;    font-size: 20px;    padding: 4px;    padding-left: 4px;    padding-right: 4px;  }  tr {    margin-bottom: 10px;    margin-top: 10px;  } </STYLE><table><thead></thead><tbody>
<tr><td>hello</td><td>world</td></tr>
</tbody></table></div></html>

The below is the code the calls it in a separate Objective c++ (.mm) file:

-(NSString *)getFieldsMapAsHtmlForResult:(Result *)result {
    const char *s = _sqliteConnection->htmlTest();
    NSLog(@"Before Conversion: %s", s);
    NSString *htmlString = [NSString stringWithUTF8String:s];
    NSLog(@"After Conversion: %@",htmlString);
    return htmlString;
}

I have seen a lot of people recommend using stringWithUTF8String to convert a c string to an NString. This has worked for me in the past but for some reason, in both the NSLog outputs, I get nothing returned. The string just mysteriously disappears.

Could you recommend what might be causing this?

----- UPDATE ---- Following Retired Ninja's Advice, I tried to make sure that the pointer referenced wasn't to a variable that fell out of scope. I had a Result model c++ class that I was passing on to my obj c++ code successfully. I decided to add a data member, fields_map_as_html, and have the function pass to that instead of to my obj-C++ code.

Was it helpful?

Solution

SqliteConnection::htmlTest() is returning a pointer to a local variable that goes out of scope so it is no longer valid. You could return a `std::string instead and extract the data in the calling function or use some other method to make sure the data hangs around until you use it.

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