Question

I’m using the initializer list method for my class constructor. I initialize my variables by calling a getData member function of the dataRegistry Class which provides the data to load into my class’ variables. The getData method is normally called by passing a character array which contains the name of the data to be loaded.

For e.g. an initializer list looks like the following:

    dataItem1(dr.getData("Pop.Stats1")),
    dataItem2(dr.getData("Pop.Stats2"))

However, due to the way the names of provided, I need to pull the name of the dataset from a user class and then append the variable name to pass to the getData method. Something like this:

    datasetName(user.getAttributeString("DataSetName")),
    dataItem1(dr.getData(datasetName + ".Stats1")),
    dataItem2(dr.getData(datasetName + ".Stats2"))

where datasetName is defined as a std::string variable.

However, this throws the following error:

     no matching function for call to 'DataRegistry::GetData(std::basic_string<char,std::char_traits<char>, std::allocator<char> >)'

Since the implementation of the getData method is not provided to me and I know that it works when I just pass a char array (e.g.”Pop.Stat1″) to the function, I suspected this was because the function accepted character arrays only. So, I tried:

    datasetName(user.getAttributeString("DataSetName")),
    dataItem1(dr.getData(strcat(datasetName.c_str() , ".Stats1"))),
    dataItem2(dr.getData(strcat(datasetName.c_str() , ".Stats2"))),

but this throws the error:

    error: invalid conversion from 'const char*' to 'char*'
    initializing argument 1 of 'char* strcat(char*, const char*)'

I’m clueless as to what’s going wrong? Please advise!

Answer 1

Almost, though using strcat on the return value of c_str() causes undefined behaviour. Just do:

(datasetName + ".Stats1").c_str()

so the whole line looks like

dataItem1(dr.getData((datasetName + ".Stats1").c_str())),

Because you’re concatenating ".Stats1" onto datasetName via string::operator+, then you want to get the const char* to that string, so you use c_str() on the return value of operator+.

You need the parentheses around it because . has higher precedence than +, so if the parens weren’t there, you’d be calling c_str() on “.Stats1” (instead of the return value of +) which doesn’t exist.