How can I convert between enum and cstring without map? - c++

Question

I've seen this answered before but using map or vectors, but I can't use outside libraries for my project, so I need to figure out another way. For converting to a cstring I'm using a function with a switch case and it works, but for converting from a cstring to the enum isn't going as planned.
The method I came up with for converting a cstring to an enum is to first cast the enum to an int (none, first, second, etc.. becomes 0, 1, 2, etc...) so that I can use a for loop to iterate through the different enums. Next, using the enum to cstring function, I compare the string passed in to the string given by the converting function. If they are equal, the enum is set. This seems like a pretty convoluted way to do this, and unsurprisingly, I can't get it working.
Here's all my test code, the setType function is where things go wrong.
enum type { none, first, second, third, fourth };
const char* typeName(type name);
type setType(char* name); // trouble here
int myStrComp(const char *str1, const char *str2); // compare cstrings
int main() { // test the function
char testName[] = "second";
type testType = setType(testName);
std::cout << typeName(testType) << std::endl; // should print "second"
}
const char* typeName(type name) { // convert enum to cstring
switch (name) {
case none: return '\0'; break;
case first: return "first"; break;
case second: return "second"; break;
case third: return "third"; break;
case fourth: return "fourth"; break;
}
}
type setType(char* name) {
type temp;
for (int i = 0; i < 4; i++) { // I know, inefficient
temp = static_cast<type>(i); // but there's only 5 to check
if (myStrComp(name, typeName(temp)) == 0) {
return temp;
}
}
return none; // shouldn't get here
}
int myStrComp(const char *str1, const char *str2) {
while (*str1 == *str2) {
if (!*str1) {
return 0; // strings are equal
}
str1++;
str2++;
}
return *str1 - *str2; // how different are they alphabetically
}

Answer 1

case none: return '\0'; break;
This has single quotes, so it returns character \0, which as an integer is equal to 0. When converted to pointer, this is a null pointer. When you try to dereference a null pointer in myStrComp(), an access violation happens.
Instead, you can use return ""; to return an empty string.
Possible way to simplify typeName is to use an array:
const char* typeName[] = {"", "first", "second", "third", "fourth"};
if (myStrComp(name, typeName[i]) == 0)
(This will cause access violation if i is out of bounds.)

Answer 2

To answer your question, you can associate enums with text by using a lookup table:
struct Entry
{
type enum_type;
const char * enum_text;
};
Entry enum_conversion_table[] =
{
{none, "none"},
{first, "first"},
{second, "second"},
{third, "third"},
{fourth, "fourth"},
};
static const size_t conversion_table_capacity =
sizeof(conversion_table) / sizeof(conversion_table[0]);
Converting from enum to text:
Search the table for an entry with the key enum.
Return a pointer to the text field of the entry, if found, or nullptr if not found.
Converting from text to enum:
Search the table for an entry with the key enum text.
Return the enum value if found, or create another enum value for "unknown" and return that.
This technique:
1. Doesn't use any libraries.
2. The data can be placed into the constant data section and stored into read-only memory.
3. The code can access the data directly.
4. The data is initialized before main().