将一个 Char 数组添加到 C++ 中的 Char * 变量

Question

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.

Answer 1

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:

• https://www.thecodingdelight.com/string-cplusplus/
• How to concatenate two strings in C++?
• https://docs.microsoft.com/en-gb/cpp/text/string-and-io-formatting-modern-cpp?view=vs-2019

Answer 2

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() 

Answer 3

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) );
}