std::string as struct member variable

[hugob0ss]

Hi, i'm with a problem here that i can't understand what it is.

Hi have this code

 struct SF {
 
  std::string mnemonic;//mnemonic that represents it
  std::string name;//a descriptive name
  ushort num_val1;// number of the first value
  uchar possiblevalues_num; //number of the possible values.
  uchar type; /*  = 1 if the feature is utilized in the models*/
};

basically i am trying to declare a std::string object in a struct to later fill it with information

the next step i allocate an array of SF structs and try to do it

 SF * FTab;
  FTab = (SF *) calloc(6, sizeof(SF));
 
 
  FTab[0].num_val1 = (ushort) 5;
  FTab[0].mnemonic("xpto");

the ushort type i can acess, the std::string generates a segmentation fault error, and i can't understant why. Does anyone know what i am doing wrong?

PS- sorry for the bad english.

[weaknessforcats]

It's the call to calloc.

C++ uses constructors and destructors to initialize and clean up objects. calloc doesn't know about this so never called them. Your string members are garbage.

Do not use C memory allocation in C++.

Use only new an delete. Here is the corrected code:

struct SF {
 
  std::string mnemonic;//mnemonic that represents it
  std::string name;//a descriptive name
  unsigned short num_val1;// number of the first value
  unsigned char possiblevalues_num; //number of the possible values.
  unsigned char type; /*  = 1 if the feature is utilized in the models*/
};
 
int main()
{
SF * FTab;
  FTab = new SF[5];
 
  FTab[0].num_val1 = 5;
  FTab[0].mnemonic ="xpto";
}
 

Decide now whether you will write in C++ or not. If yes, then do not:
1) malloc, calloc, alloc, etc.....
2) memcpy, mem... anything
3) free
4) exit(1)
5) strcpy, str... anything

for openers.

[hugob0ss]

It's the call to calloc.

C++ uses constructors and destructors to initialize and clean up objects. calloc doesn't know about this so never called them. Your string members are garbage.

Do not use C memory allocation in C++.

Use only new an delete. Here is the corrected code:

struct SF {
 
  std::string mnemonic;//mnemonic that represents it
  std::string name;//a descriptive name
  unsigned short num_val1;// number of the first value
  unsigned char possiblevalues_num; //number of the possible values.
  unsigned char type; /*  = 1 if the feature is utilized in the models*/
};
 
int main()
{
SF * FTab;
  FTab = new SF[5];
 
  FTab[0].num_val1 = 5;
  FTab[0].mnemonic ="xpto";
}
 

Decide now whether you will write in C++ or not. If yes, then do not:
1) malloc, calloc, alloc, etc.....
2) memcpy, mem... anything
3) free
4) exit(1)
5) strcpy, str... anything

for openers.

thanks it worked!
i am not very comfortable also writing code this way, but i'm working in a legacy system in which the memory management is made in C, so i assumed that my program would also have the same structure.

[weaknessforcats]

but i'm working in a legacy system in which the memory management is made in C, so i assumed that my program would also have the same structure.

In that case I would stick with C entirely. Mixing C++ and C is not a good idea unless you clearly know what you are doing.

[Alex340]

Thanks a lot for this answer!