There is a structure that has multiple const char* member variables.
typedef struct WWCryptoAuthorizationHeader
{
int age;
const char *name;
const char *address;
} StudentInfo;
There is a function that takes a (pointer to) this struct and a (C-style) string containing the name of one of the member variables. Within this function, I create a map from member names to their values. Then I find the provided member name in the map. Can I have the function change the designated member of the struct using the value from the map?
I tried the following
void foo(StudentInfo *studentInfo, const char* key)
{
std::map<std::string, const char*> studentInfoMap;
studentInfoMap.insert(std::make_pair("name", studentInfo->name));
studentInfoMap.insert(std::make_pair("address", studentInfo->address));
for(auto &item : studentInfoMap)
{
if (strcmp(key, item.first.c_str()) == 0)
{
item.second = "John";
}
}
}
but *studentInfo was not updated.
4
You are storing a copy of the value of your const char* pointers into your map, so you are not able to update the original pointers inside the StudentInfo struct. You would have to store the address of the pointers instead, eg:
void foo(StudentInfo *studentInfo, const std::string &key)
{
std::map<std::string, const char**> studentInfoMap;
studentInfoMap.insert(std::make_pair("name", &(studentInfo->name)));
studentInfoMap.insert(std::make_pair("address", &(studentInfo->address)));
for(auto &item : studentInfoMap)
{
if (key == item.first)
{
*(item.second) = "John";
}
}
}
That being said, the for loop is unnecessary. You can use map::operator[] or map::at() to access an element by its key, eg:
void foo(StudentInfo *studentInfo, const std::string &key)
{
std::map<std::string, const char**> studentInfoMap;
studentInfoMap.insert(std::make_pair("name", &(studentInfo->name)));
studentInfoMap.insert(std::make_pair("address", &(studentInfo->address)));
*(studentInfoMap[key]) = "John";
// or
*(studentInfoMap.at(key)) = "John";
}
You should also consider using std::string instead of const char* inside of your struct, eg:
struct StudentInfo
{
int age;
std::string name;
std::string address;
};
void foo(StudentInfo *studentInfo, const std::string &key)
{
std::map<std::string, std::string*> studentInfoMap;
studentInfoMap.insert(std::make_pair("name", &(studentInfo->name)));
studentInfoMap.insert(std::make_pair("address", &(studentInfo->address)));
*(studentInfoMap[key]) = "John";
// or
*(studentInfoMap.at(key)) = "John";
}
1