Access global data structure - c++

I'm new to C++ and trying to make a map that all the source files will be able to access. Here is a simplified version of the problematic code. Every header file has a header guard, I just didn't type them here.
// main.cpp
#include "client.hpp"
int main(void){
init();
search();
}
// util.cpp
#include "util.hpp"
std::map<int, STUDENT_TYPE> dataBase;
init(){
dataBase[0] = STUDENT_TYPE(14, 4.0);
// more students....
}
// util.hpp
#include <map>
struct STUDENT_TYPE{
int age;
int grade;
STUDENT_TYPE(int age, int grade) : age(age), grade(grade){}
};
extern std::map<int, STUDENT_TYPE> dataBase;
// client.cpp
#include "client.hpp"
void search(){
std::cout << dataBase[0].grade << std::endl;
}
// client.hpp
#include "util.hpp"
void search();
The problem is that the compiler failed to build at the search function. It gives a big chain of errors. The last error or the cause of all the error is that the constructor of STUDENT_TYPE require 2 fields while given 0. I suspect that client can't access the STUDENT_TYPE struct inside the dataBase. I don't how to fix it or exactly how it happened. I just want a big table of students that all the files in the program can access.

The compiler is telling you exactly what's wrong. Your student type doesn't have a default constructor and it's being used.
Why is it being used?
Because map::operator [] will create a default constructed instance to return if an entry doesn't yet exist for that key. Even if this never happens, the compiler still has to compile that branch and it can't here.
Two ways to fix:
Give your student type a meaningful default.
Don't use map::operator[]. Instead use map::find.

First solution; don't have a global data structure. There is normally no need at at all to have such a thing.
Secondly, understand what header guards do - they prevent the body of a header file being included in the same translation unit. They have no effect if the header file is included in multiple .cpp files.
Thirdly, when you post questions about problems here, post the full text of the error messages you are getting.

Related

Use one globe variable in all cpp files.If one class of the cpp file changed the value,I want to access it from another class cpp file

I want to Use one globe variable in all cpp files.If one class of the cpp file changed the value,I want to access it from another class cpp file,which is the value that least modified by any other cpp class file.
str.h - global variable file
#ifndef MY_PROG
#define MY_PROG
extern char * myname;
#endif
class1.cpp
#include "str.h"
char * myname;
class class1
{
class1(){}
~class1(){}
void Setname1(char *name) { myname = name }
};
class2.cpp
#include "str.h"
char * myname;
class class2
{
class2(){}
~class2(){}
void setName(char *name) { myname = name }
};
class3.cpp
#include "str.h"
class class3
{
class3(){}
~class3(){}
char *GetData()
{
return myname;
}
};
main.cpp
#include "str.h"
int main()
{
class1 c1;
class2 c2;
c1.Setname1("XXXX");
c2.setname("YYYY");
class3 c3;
cout << c3.GetData;
}
when I execute the program, I need to get Last modified value that is "YYYY" .I am new to cpp, And also please tell me whether I used the extern keyword correctly.If not , please provide me the right procedure.
The essential in your problem is understanding the difference between declarations and definitions of variables (and types, functions etc. objects in C/C++)
extern char * myname; is a declaration. It makes myname visible to other pieces of code in the same file so that they can reference it.
char * myname; is a definition. Not only it makes myname visible, it also instructs the compiler to allocate space for it and to make its address known.
You can have as many declarations of the same variable in your code as you want, as long as they do not contradict each other. No so with definitions. If you define a thing two times, it would need to be assigned two addresses, and then how can other object files "understand" which address to use when referencing it? The same goes with the space allocated — what to do with the extra piece of it allocated? This is the error that you see.
To make the code work, have only one and exactly one definition of myname in exactly one file. It must not be a header file because it gets copied into multiple source files thus creating multiple definitions. It can be any other C++ file though.
In the rest of the files (or in a single header included in all of them) have multiple declarations of myname if it is referenced in a particular file. If not, you can omit it for a particular unit.
All this being said, it is considered to be a VERY BAD PRACTICE to communicate data between compilation units through global mutable shared variables. They make code a nightmare to debug and understand and impossible to parallelize. Nobody ever thinking getting money for the code they write should use them. Instead, the best approach would be to pass a mutable object as one of methods/functions argument. Details actually depend on the rest of your application.

setter in class won't set variable

I'm currently trying to make a game in C++. In my code I'm trying to nest my variables so that my main doesn't have a lot of includes. My problem right now though is that the value of my variables in my class aren't changing. Stepping through the code it shows it setting the value, but it doesn't work. Anyone know what's going on? Thank you in advance.
This is what I have so far:
Location.h
#ifndef LOCATION_H
#define LOCATION_H
#include <string>
class Location
{
public:
Location(void);
Location(std::string name);
~Location(void);
std::string GetName();
void SetName(std::string value);
private:
std::string m_Name
};
#endif
Location.cpp
#include "Location.h"
Location::Location(void): m_Name("") {}
Location::Location(std::string name): m_Name(name) {}
Location::~Location(void)
{
}
std::string Location::GetName()
{return m_Name;}
void Location::SetName(std::string value){m_Name = value;}
PlayerStats.h
#ifndef PLAYERSTATS_H
#define PLAYERSTATS_H
#include "Location.h"
class PlayerStats
{
public:
PlayerStats(void);
~PlayerStats(void);
Location GetLocation();
void SetLocation(Location location);
private:
Location m_Location;
};
#endif
PlayerStats.cpp
#include "PlayerStats.h"
PlayerStats::PlayerStats(void): m_Location(Location()) {}
PlayerStats::~PlayerStats(void)
{
}
Location PlayerStats::GetLocation(){return m_Location;}
void PlayerStats::SetLocation(Location location){m_Location = location;}
main.cpp
#include <iostream>
#include "PlayerStats.h"
using namespace std;
PlayerStats playerStats = PlayerStats();
int main()
{
playerStats.GetLocation().SetName("Test");
cout<< playerStats.GetLocation().GetName()<<endl;
return 0;
}
Your immediate issue is that
Location GetLocation();
returns a copy of the location, so when you call SetName here:
playerStats.GetLocation().SetName("Test");
You're changing the name of the temporary copy, and the change is lost as soon as the semicolon is hit.
More broadly, this kind of design (nesting classes and nesting includes so that main doesn't have a lot of includes, and using a.b.c() style code to access nested members) isn't great C++ style:
Having a bunch of source files that (transitively) include a bunch of header files means that changing a single header file will trigger recompilations of a bunch of source files. Compile times can be a significant issue in larger C++ projects, so reducing compile times by controlling #include's is important. Read up on "forward declarations" for more information.
Writing code like a.b.c() is considered bad object-oriented design, because it reduces encapsulation: not only does the caller have to know about a's details, it has to know about b's also. Sometimes this is the most expedient way to write code, but it's not something to be blindly done just to reduce #include's. Read up on "Law of Demeter" for more information.
If you want to set the result of playerStats.GetLocation(), you could make GetLocation() pass-by-reference (use ampersand, &, on the return argument). Otherwise you are just setting values in a temporary copy of PlayerStats::m_Location.
Alternatively, you could use the SetLocation() function.

My Function Doesn't Know My Struct Values (c++)

I'm new to doing structs, so please bear with me if this turns out to be a dumb question. I have one header file and four .cpp files that all include it. I have a struct called ToDoLista and it has string nameIt and int DeadLine. Then I have the things whose type name I don't know that are like, the Soccer and DropOffMax and stuff.
ToDoLista Soccer, DropOffMax, CookDinner;
Soccer.DeadLine=6;
Soccer.nameIt="SOCCER";
//and so on, for a total of six, 3 ints and 3 strings definitions.
This struct seems to be finnicky if I try to move it around because if it's in the header it's included three times and it wont run due to 'multiply defined' whatever. If I put it in one of my three non-main cpp files, it seems that the struct won't work because some of it has to be defined in main(). So now it's in my main cpp file, but I have functions that use these values, and those functions are in my non-main cpp files, which as far as I know compile before the main one. To get around that, I put the struct declaration in the header, and the definitions in my main (I may have mis-worded that) AND THEN I say 'okay, run the function 'CheckItTwice'.
//MAIN
Soccer.DeadLine=6;
//and so on for all six, like before.
//ok, NOW run the fx.
CheckItTwice(Soccer.Deadline, Soccer.nameIt);
The issue here is that if I tell CheckItTwice to say, cout the string, or the int, it runs the program without errors, but returns nothing in the console where the cout should be, because apparently they haven't been defined yet, as far as the function is concerned. Why is this/do you know a way around this?
In order to avoid the "defined multiply" errors you need to define your struct in a header file, and put a #pragma once or #ifndef...etc block at the top. See this here.
Include the header file in any implementation (cpp) file you plan to use the struct in.
The line
ToDoLista Soccer, DropOffMax, CookDinner;
declares three instances of the struct ToDolista, called Soccer, DropOffMax, and CookDinner. They are not types, they are instances of a type, that type being ToDolista.
I can't comment on the contents of CheckItTwice() as you didn't provide them, but look here for guidance on using cout. You might want to consider passing the struct as one argument to this method, preferrably as a const reference.
Define the struct in your header and #include that header in the cpp files. In the header try adding
#pragma once
at the top of your header file. This is a Microsoft specific extension - documented here
The more portable version is to add
#ifndef _SOME_DEF_
#define _SOME_DEF_
struct ToDoLista {
string namit;
string project;
int status;
int speed;
int difficulty;
int priority;
int deadline;
}
#endif // _SOME_DEF_
Be sure to remove struct definition from the .cpp file.
I got this issue resolved using something I saw while searching desperately online: extern. it seems that if I put the declaraction in the header, the definition in a cpp, and then declare the object again in another cpp but with 'extern' before it, it works like a charm as far as keeping the values from the original definition.
Header.h
struct ToDoLista{
string namit;
string project;
int status;
int speed;
int difficulty;
int priority;
int deadline;
};
Side.cpp
ToDoLista Pushups, Tumblr, Laundry, CleanRoom, CleanHouse, Portfolio;
void CheckItTwice(int staytus, string name){
if(staytus==1){//not on the list
staytus==2;
cout << "hurry up and" << name << " okay?" << endl;
Main.cpp
extern ToDoLista Pushups, Tumblr, Laundry, CleanRoom, CleanHouse, Portfolio;
Pushups.namit = "Push Ups";
Pushups.status = 1;
Pushups.speed = 0;
Pushups.difficulty = 0;
Pushups.priority = 0;
Pushups.project = "Get Fit";
Pushups.deadline = 20131102;
CheckItTwice(Pushups.status,Pushups.namit);
This works for me, and I hope this 'answer' helps someone else.

List Variable in Library Causes run time error, why?

OK folks. I have fixed the error by moving the variable definition, but I do not understand why there is a problem.
Simplified Background: I have an object and I want to track all instances of that object in a list, so I simply created a List<> static member of the class. Below was a simple representation that allowed me to play with it. If I have the line marked as "this line" in the static library. I get a run time error. The object is defined in a header file and is the same header file in both places. If I move "this line" to the code in my final application and it works.... Why? I just don't understand why it is different.
#include "stdafx.h"
#include <list>
using namespace std;
class someobject
{
public:
someobject()
{
// do some stuff.
theStaticList.push_back(this);
}
void func()
{
printf("Made it!!\n");
}
static list<someobject*> theStaticList;
};
list<someobject*> someobject::theStaticList; //*** This line
someobject global;
int main()
{
someobject initial;
initial.func();
global.func();
list<someobject*>::iterator iter;
printf("\n\nLoop the Static List\n");
for (iter = someobject::theStaticList.begin(); iter != someobject::theStaticList.end (); iter++)
(*iter)->func();
return 0;
}
If you put that line in a header file, then include the header into two or more source files, you're defining the list object in each source file where the header gets included.
This violates the one definition rule, so the linker will quite rightly give you an error when you do it.
You want to define the object in one (and only one) source file. For a library, that should be some object file in the library, not the user's source file though (at least as a general rule).

Difference between two header orderings in C++ seems to generate error

I have a class called File that is defined (along with other classes) in the header "dmanager1.h". In the "dmanager1.cpp" file (implementation for the dmanager1.h file), when I list the headers in one order I get an error when trying to compile along with my main.cpp (main.cpp is empty except for the header call and an empty "int main()"...basically I'm just testing the class .h and .cpp files)... If I switch the headers around in the dmanager1.cpp file I get no errors. I don't understand what is happening. The error I'm getting is:
error: 'File' does not name a type
I get said error when I have my header's ordered in my "dmanager1.cpp" as follows:
#include "dmanager1.h"
#include <iostream>
#include <cstring>
If I switch the header's around to:
#include <iostream>
#include <cstring>
#include "dmanager1.h"
...I don't get the compilation error. Is the first order getting parsed funny? Any thoughts would be greatly appreciated.
EDIT: Added part of the header in question...
#ifndef _dmanager1_h
#define _dmanager1_h
//--------------------
// Forward References
//--------------------
// Node_L, Node_T, and Sector are defined in File: dmanager1a.h
class Node_L;
class Node_T;
class Sector;
class File
{
public:
// Default Constructor
//File();
// Constructor: Allowing "name", "size", and/or "permissions" to be set
// Permissions set to default of 0 == read and write
File(const char * & name, float size = 0, int permissions = 0) : timestamp(11223333) {};
// Default Destructor
~File();
//returns an int corresponding to the date modified (mmddyy)
int get_date_mod(void) const {return timestamp;}
// Return's current level of permission on the File: 0 = read/write, 1 = read only
int get_permission(void) const {return permission;}
// Set's Permission to "level": 0 = read/write, 1 = read only
int set_permission(int level);
private:
// Data members
char * name;
float size_OA;
//function used to update "date modified"
void update_timestamp(void);
// Current permission level of the file: 0 = read/write, 1 = read only
int permission;
//value modified by update_timestamp() and the value returned by get_date_mod(). Date file last edited.
int timestamp;
};
Most likely your dmanager1.h header needs something that iostream or cstring define.
As a result, it doesn't get parsed correctly, and the compiler doesn't understand the declaration of your File class.
If you post your dmanager1.h file, you'll be able to get a more detailed answer.
Make sure that each of your headers is completely self-sufficient. It needs to #include headers for everything that it uses and not assume that they will be included by something else. Every header should work even if it is the only header that a .c file includes.
I'm betting that your dmanager1.h header is using something from the standard library and you aren't including the header that it needs. Swapping the header appears to fix the problem, but it's only working by coincidence.
One diagnostic test you can do is to create a .c file that contains nothing but the line #include "dmanager1.h". Try to compile it. If the compiler throws an error, it should provide hints as to which additional headers need to be included.
Update: I can compile using the initial portion of the header that you posted using g++ -Wall and I get no errors or warnings at all. Please post a sample that reproduces the problem.