Casting std::vector<std::string> to void*

Question

I'm trying to pass in a std::vector<std::string> to a void* for use in a callback function which requires a void* to be passed to it. The callback is a sqlite_exec callback. However, I am getting a segfault when casting vec to a void* . So how can I pass in a std::vector<std::string> into such a callback?

static int cb_vector(void* vec, int argc, char**argv, char**colNm){
    std::vector<std::string>* myvec = (std::vector<std::string>*)vec;
    for(int i = 0; i < argc; i++){
        myvec->push_back(argv[i]);
    }
    return 0;
}

std::vector<std::string> myfunc(){
    std::vector<std::string> vec;
    int rc = sqlite3_exec(_db, SQL, cb_vector, (void*)&vec, NULL);
    return vec;
}

I have already looked the following resources, but they did not answer my quesion.

• Converting (void*) to std::vector<unsigned char>
• Passing a vector to a function as void pointer

Answer 1

First of all void * specially designed to accept pointer to any data, so your explicit cast to it is not necessary. Second - you better use C++ cast in C++ code:

std::vector<std::string>* myvec = static_cast<std::vector<std::string>*>( vec );

At least it is easier to catch problems later when you use keyword for your cast.

According to sqlite3_exec documentation:

The 3rd argument to the sqlite3_exec() callback is an array of pointers to strings obtained as if from sqlite3_column_text(), one for each column. If an element of a result row is NULL then the corresponding string pointer for the sqlite3_exec() callback is a NULL pointer.

Most probably you get nullptr and try to push it into vector . Put additional check there:

myvec->push_back(argv[i] ? argv[i] : std::string() );

or deal with it accordingly