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I am makinga code to send a query for my SQLlite database, but I just can't find a way to add a variable to the Char * variable that is being used as the query. the query and the variable that I want to add are declared like this:

String p = "10004F1F7";
 char *sql = "SELECT * from TABELTAGGEGEVENS WHERE ID =" + p;

the error i get is this: error: invalid user-defined conversion from 'Arp::BasicString' to 'char*' [-fpermissive]

many thanks.

RoyCL14
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    What's `String`? Why is the `"String p = ` inside the string as well? – Blaze Nov 26 '19 at 09:21
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    Why do you think *adding* a literal string to an object of whatever `String` is would result in a `char *`? – Some programmer dude Nov 26 '19 at 09:22
  • what is `Arp::BasicString` ? Did you consult its documentation? Does it have a conversion to a c array of `char`? – 463035818_is_not_an_ai Nov 26 '19 at 09:22
  • How are you expecting the memory holding this string to be managed? Also, what's a `String` exactly? Is it coming from some library you're using? – David Schwartz Nov 26 '19 at 09:44
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    Make sql an std::string, and add the two strings with std::string's operator+ If, for some reason, you need to pass sql as a char*, use std::string's c_str(). Note it returns a const char*, if you need a non-const pointer, check whether the function actually changes the string, in which case you'll have to copy it to a char array, otherwise you could const_cast (ugly as it is). – Uri Raz Nov 26 '19 at 09:50

3 Answers3

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This doesn't work because in C++ the '+' operator on char pointers doesn't concatenate the strings. One solution would be to make the literal value a String as well:

String p = "10004F1F7";
String query = "SELECT * from TABELTAGGEGEVENS WHERE ID =";

You can then concatenate like this: + operator: p + operator

I don't know the particular library you appear to be working with (Arp::BasicString), so I don't know how you'd convert that into a char *.

With std::string you can simply call c_str on the result.

Another and probably better solution is to use formatters.

For reference see:

devb
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String p = "10004F1F7";
char *sql = "SELECT * from TABELTAGGEGEVENS WHERE ID =" + p;

You can handle the Arp::BasicString almost like an std::string.

Looking through the headers you will find a Method named "CStr()".

Like that you should be able to do something like this:

String P = "10004F1F7";
String Query = "SELECT * from TABELTAGGEGEVENS WHERE ID =" + p;
char *sql = Query.CStr()
marc_s
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Oliver Warneke
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  • This is even better than the accepted answer, because it gives a simple explanation for how to convert the resulting string to `char *` – Martin Dec 28 '19 at 23:54
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The only hit on Google for Arp::BasicString was used in the SDK for a software company, PLCnext. A little bit of riffling through I found a header file BasicString.hxx and inside a prototype for BasicString class template. There the baseString data structure is private.

I had to come up with this (rather low-level) workaround, compiling with PLCnext software succeeded and passed tests when adjusted for std::string):

String p = "10004F1F7";
const char* CMD_SEQUENCE = "SELECT * from TABELTAGGEGEVENS WHERE ID =";
const int CMD_LENGTH = 41;

// allocate and assign memory for the static characters in the command
char *sql = (char *)malloc(CMD_LENGTH * sizeof(char));
memcpy(sql, CMD_SEQUENCE, CMD_LENGTH);

// iterate through all chars in String p
// resizing the memory buffer as needed and adding ith char to the end
for (int i=0; i<p.Size();i++){

    sql = (char*)realloc(sql, (CMD_LENGTH + i) * sizeof(char));

    // destination is the ith memory cell past the cmd sequence
    int destIdx = CMD_LENGTH + i;

    // copy 1 char at a time; ith char in p
    memcpy( &sql[destIdx], &p.At(i), sizeof(char) );
}
jackw11111
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