Error: Cannot add two pointers

Question

It gives the error in the title about this piece of code:

string DDateTime::date2OracleDate(DATE Date)
{
    string s;
    s="TO_DATE('" + DateFormat("%d/%m/%Y",Date) + "','dd/MM/YYYY')";
    return s;
}

I don't understand how that is possible, no pointers involved....

EDIT:

string DDateTime::DateFormat(string sFormat,DATE Date)
{
    struct tm tmTemp;
    RipOf_AfxTmFromOleDate(Date, tmTemp);
    RipOf_AfxTmConvertToStandardFormat(tmTemp);
    char sFormatted[MAX_TIME_BUFFER_SIZE];
    strftime(sFormatted, MAX_TIME_BUFFER_SIZE, sFormat.c_str(), &tmTemp);
    return sFormatted;
}

Answer 1

Your literal strings are char*. I am not sure what your string is, but replacing each quoted string with a constructed string should do it.

s=string("TO_DATE('") + DateFormat("%d/%m/%Y",Date) + string("','dd/MM/YYYY')"); 

Answer 2

s="TO_DATE('" + DateFormat("%d/%m/%Y",Date) + "','dd/MM/YYYY')";

means (to the compiler) that you are adding a const char* pointer to something else (possibly a char*?) returned by / converted from DateFormat, and then add to it another const char* pointer.

Try this to force your compiler to find the correct string overloads for operator+:

s=string("TO_DATE('") + DateFormat("%d/%m/%Y",Date) + "','dd/MM/YYYY')";

Answer 3

The following should work better:

string DDateTime::date2OracleDate(DATE Date)
{
    string s = "TO_DATE('";
    s += DateFormat("%d/%m/%Y",Date);
    s += "','dd/MM/YYYY')";
    return s;
}

Answer 4

The three strings you're trying to add are C-style strings; each is a pointer to the contents of the string. At least, I'm assuming that DataFormat returns a C-style string; it's not a standard function, so I don't know what it does.

In C++ you can't simply "add" two of these together to get a C++ string, since there's no way of knowing whether a char* is actually a string, or a pointer to some arbitrary data. You have to convert one of them into a std::string, then you can "add" C-style strings to that.

string s = "TO_DATE(";
s += DateFormat(whatever);
s += "','dd/MM/YYYY')";

or

string s = string("TO_DATE(") + DateFormat(whatever) + "','dd/MM/YYYY')";

Answer 5

The probelm is only coming from the return type of DateFormat being char* - std::strings can normally be concatenated with string literals without any problem.

E.g. the following works:

std::string a, b;
b = "foo";
a = "literal" + b + "literal";

but the following will not work:

std::string a, b;
b = "foo";
a = "literal" + b.c_str() + "literal";

The only change required should be to convert the result from DateFormat to a std::string.

s="TO_DATE('" + std::string(DateFormat("%d/%m/%Y",Date)) + "','dd/MM/YYYY')";

Should do the trick.

Edit 1

Seeing that DateFormat returns string there should have been no problems concatenating literal + std::string + literal. So I'm stumped.

Edit 2

Following up after Amardeep's noticing that DateFormat actually returns a char* despite the signature suggesting otherwise (nice catch). I tested and it indeed appears no errors or warnings are raised, even compiling with -Wall and -Wextra on gcc, the function behaves as if it were simply defiend char*.

So my solution should still work for the initial reasons. (and there is no actual problem with concatenating string literals with std::string, the problem is only char* with string literals).

It would probably be a neater solution to fix the root of the problem - the bad return type of DateFormat by converting sFormatted to a std::string.

    return std::string(sFormatted);