C++ - Passing string value to a function using uint8_t pointer - c++

Question

I am learning C++ in order to create a custom function (user defined function is how cloudera call it) that I want to use in Hadoop Cloudera Impala SQLs. Cloudera have provided a header file that has type definitions for custom function arguments
struct AnyVal {
bool is_null;
AnyVal(bool is_null = false) : is_null(is_null) {}
};
//Integer Value
struct IntVal : public AnyVal {
int32_t val;
IntVal(int32_t val = 0) : val(val) { }
static IntVal null() {
IntVal result;
result.is_null = true;
return result;
}
}
//String Value
struct StringVal : public AnyVal {
static const int MAX_LENGTH = (1 << 30);
int len;
uint8_t* ptr;
/// Construct a StringVal from ptr/len. Note: this does not make a copy of ptr
/// so the buffer must exist as long as this StringVal does.
StringVal(uint8_t* ptr = NULL, int len = 0) : len(len), ptr(ptr) {
assert(len >= 0);
};
/// Construct a StringVal from NULL-terminated c-string. Note: this does not make a copy of ptr so the underlying string must exist as long as this StringVal does.
StringVal(const char* ptr) : len(strlen(ptr)), ptr((uint8_t*)ptr) {}
static StringVal null() {
StringVal sv;
sv.is_null = true;
return sv;
}
}
Now for a simple Add function like the one below I understood how to pass the reference of IntVal object after setting IntVal.val and it worked !
IntVal AddUdf(FunctionContext* context, const IntVal& arg1, const IntVal& arg2) {
if (arg1.is_null || arg2.is_null) return IntVal::null();
return IntVal(arg1.val + arg2.val);
}
int main() {
impala_udf::FunctionContext *FunctionContext_t ;
IntVal num1, num2 , res;
num1.val=10;
num2.val=20;
IntVal& num1_ref = num1;
IntVal& num2_ref = num2;
res = AddUdf(FunctionContext_t, num1_ref, num2_ref);
cout << "Addition Result = " << res.val << "\n";
}
But I don't know how to do similar thing for a string function as StringVal requires me to pass pointer of uint8_t type for a string? I tried below one but then received "error: cannot convert std::string to uint8_t* in assignment"*
int main() {
impala_udf::FunctionContext *FunctionContext_t ;
StringVal str , res;
string input="Hello";
str.len=input.length();
str.ptr=&input;
StringVal& arg1=str;
res = StripVowels(FunctionContext_t, str);
cout << "Result = " << (char *) res.ptr<< "\n";
}
I also tried the following but no joy. Any pointer in the right direction will be much appreciated. Thanks.
str.ptr=reinterpret_cast<uint8_t*>(&input);

Answer 1

String itself is not a character pointer (which is what you need), but you can get one by using the c_str function.
str.ptr=(uint8_t*)(input.c_str ());
If you want to use new-style casts you might need both a const_cast (to cast from const char * to char *) and a reinterpret_cast, depending on how str.ptr is defined.

Answer 2

That's because you need a pointer to c-string, and you provide a pointer to std::string. str.ptr = input.c_str() should work for you.
EDIT:
However, it seems you need a non-const pointer. In this case you need to allocate input variable yourself, like:
char input[128];
This creates a fixed size array on the stack.
But you might want to allocate it dynamically with new:
char* input = new char[size];
Also check out functions in the cstring header, you might want to use those.
You might also need to cast it to uint8_t* as described above.
Don't forget to delete[] the string later when you don't need it anymore. But since you pass it to a function, this function should probably handle this.