C++ Global variables and initialization order - c++

Let's say, I've got a following simple code:
Main.cpp
#include "A.h"
// For several reasons this must be a global variable in the project
A a1;
int _tmain(int argc, _TCHAR* argv[])
{
// Another stuff
return 0;
}
A.h
#pragma once
#include <string>
class A
{
private:
// The following works normal if we use simple types like int and etc.
static std::string myString;
public:
A();
};
A.cpp
#include "stdafx.h"
#include "A.h"
// This executes after A::A(), so we are losing all the modifyed content
// If we skip the ="test" part, the string is going to be empty
std::string A::myString = "test";
A::A()
{
// Here myString == ""
myString += "1";
}
The problem is obvious: I cannot use static variables in a constructor of class A in this case as they don't save the changes. Although I need them in order to process some data.
Please, suggest me a solution.

It sounds like you are trying to force the initialization of the static to happen before the constructor is called. The last time I encountered this problem, the only reliable fix was to wrap the static inside a function.
Change the declaration to a function returning reference to string.
static std::string& myString();
Change the definition to a function like this:
std::string& A::myString() {
static std::string dummy = "test";
return dummy;
}
Change your constructor to say:
myString() += "1";
I do not currently have an MSFT compiler handy, so you may have to tweak this a little bit, but this basically forces on-demand initialization of static.
Here is a very short test programming demonstrating how this works:
#include <string>
#include <stdio.h>
std::string& myString() {
static std::string dummy = "test";
return dummy;
}
int main(){
myString() += "1";
printf("%s\n", myString().c_str());
}

Related

Struggling with pointers to functions and references

I am working through this problem I found on Git to brush up on some skills. Using friend is prohibited. C++ styling should be used compared to C.
Essentially, I cannot call the identify() function that belongs to the Brain member variable in my Human class. It just will not let me access it. If you can code this up, and explain where I am going wrong, that would be great.
Create a Brain class, with whatever you think befits a brain. It will have an Identify() function that returns a string containing the brain's address in memory, in hex format, prefixed by 0x.
Then, make a Human class, that has a constant Brain attribute with the same lifetime. It has an identify() function, that just calls the identity() function of its Brain and returns its result.
Now, make it so this code compiles and displays two identical addresses:
int main(){
Human bob;
std::cout << bob.identify() << "\n";
std::cout << bob.getBrain().identify() << "\n";
}
Here is what I have so far:
#pragma once
#include "Brain.h"
class Human
{
const Brain humanBrain;
public:
Human();
std::string identify();
};
#include "Human.h"
#include <iostream>
#include <string>
#include <sstream>
Human::Human()
{
this->humanBrain = new Brain;
}
std::string Human::identify()
{
Brain b = this->humanBrain.identify(); // This is essentially what I am trying to call--and I can't access it.
const Brain * ptr = humanBrain;
std::ostringstream test;
test << ptr;
return test.str();
}
#pragma once
#include <string>
#include <iostream>
class Brain
{
int age;
std::string gender;
void* ptr;
public:
Brain();
//std::string getBrain();
const std::string identify();
void setPtr(void* p);
};
#include "Brain.h"
#include <iostream>
#include <sstream>
Brain::Brain()
{
age = 10;
gender = "male";
}
const std::string Brain::identify()
{
//const Brain* bPtr = &this;
const Brain* bPtr = this;
ptr = this;
std::ostringstream test;
test << &bPtr;
std::string output = "Brain Identify: 0x" + test.str();
return output;
}
Your Human::humanBrain member is declared as type const Brain, which is correct per the instructions, however your Brain::identify() method is not qualified as const, so you can't call it on any const Brain object. This is the root of the problem that you are having trouble with.
In addition, there are many other problems with your code, as well:
Human::humanBrain is not a pointer, so using new to construct it is wrong. And, you don't need a pointer to get the address of a variable anyway. Nor do you actually need a pointer to the member at all in this project.
Human lacks a getBrain() method, so bob.getBrain() in main() will not compile, per the instructions.
Human::identify() is calling humanBrain.identify(), which returns a std::string as it should, but is then assigning that string to a local Brain variable, which is wrong (not to mention, you are not even using that variable for anything afterwards). The instructions clearly state that Human::identity() should simply call Brain::identify() and return its result, but you are not doing that.
Brain::identify() is printing the address of its local variable bPtr rather than printing the address of the Brain object that identify() is begin called on, per the instructions.
With all of that said, try something more like this instead:
Human.h
#pragma once
#include "Brain.h"
#include <string>
class Human
{
const Brain humanBrain;
public:
Human() = default;
std::string identify() const;
const Brain& getBrain() const;
};
Human.cpp
#include "Human.h"
std::string Human::identify() const
{
return humanBrain.identity();
}
const Brain& Human::getBrain() const
{
return humanBrain;
}
Brain.h
#pragma once
#include <string>
class Brain
{
int age;
std::string gender;
public:
Brain();
std::string identify() const;
};
Brain.cpp
#include "Brain.h"
#include <sstream>
Brain::Brain()
{
age = 10;
gender = "male";
}
std::string Brain::identify() const
{
std::ostringstream test;
test << "Brain Identify: 0x" << this;
return test.str();
}

initializing a static (non-constant) variable of a class.

I have TestMethods.h
#pragma once
// strings and c-strings
#include <iostream>
#include <cstring>
#include <string>
class TestMethods
{
private:
static int nextNodeID;
// I tried the following line instead ...it says the in-class initializer must be constant ... but this is not a constant...it needs to increment.
//static int nextNodeID = 0;
int nodeID;
std::string fnPFRfile; // Name of location data file for this node.
public:
TestMethods();
~TestMethods();
int currentNodeID();
};
// Initialize the nextNodeID
int TestMethods::nextNodeID = 0;
// I tried this down here ... it says the variable is multiply defined.
I have TestMethods.cpp
#include "stdafx.h"
#include "TestMethods.h"
TestMethods::TestMethods()
{
nodeID = nextNodeID;
++nextNodeID;
}
TestMethods::~TestMethods()
{
}
int TestMethods::currentNodeID()
{
return nextNodeID;
}
I've looked at this example here: Unique id of class instance
It looks almost identical to mine. I tried both the top solutions. Neither works for me. Obviously I'm missing something. Can anyone point out what it is?
You need to move the definition of TestMethods::nextNodeID into the cpp file. If you have it in the header file then every file that includes the header will get it defined in them leading to multiple defenitions.
If you have C++17 support you can use the inline keyword to declare the static variable in the class like
class ExampleClass {
private:
inline static int counter = 0;
public:
ExampleClass() {
++counter;
}
};

Empty std::string in static method initialiser

I am using a static method to initialise the const fields of a class. The static method uses some const variables that are stored in a separate header file. Primitive types are correctly being passed to the static method, but the std::strings are being passed empty. I cannot understand why this is.
After doing some searching I have stumbled upon something called the static initialiser fiasco, but I'm having trouble wrapping my head around it, and can't work out if it is to blame. As the the object is in global scope, is the problem that it is being 'setup' before the std::string class has been 'setup'?
I have tried to replicate a minimal example below:
// File: settings.hpp
#include <string>
const std::string TERMINAL_STRING "Printing to the terminal";
const std::string FILE_STRING "Printing to a file";
// File: printer.hpp
#include <string>
#include <iostream>
class Printer
{
private:
const std::string welcomeMessage;
static std::string initWelcomeMessage(std::ostream&);
public:
Printer(std::ostream&);
}
extern Printer::print;
// File: printer.cpp
#include "settings.hpp"
std::string Printer::initWelcomeMessage(std::ostream &outStream)
{
if (&outStream == &std::cout)
{
return (TERMINAL_STRING);
}
else
{
return (FILE_STRING);
}
}
Printer::Printer(std::ostream &outStream) :
message(initWelcomeMessage(outStream)
{
outStream << welcomeMessage << std::endl;
return;
}
// File: main.cpp
#include "printer.hpp"
printer print(std::cout);
int main()
{
return (0);
}
Thanks very much!
As the the object is in global scope, is the problem that it is being 'setup' before the std::string class has been 'setup'?
Yes.
Have your strings be function-statics, returned by reference from some function, instead.
This is the traditional fix for the static initialisation order fiasco.

Enum in a class with strings

I'm trying to implement a class (C++) with an enum (with the permitted parameters). I got a working solution, but if I try to extend the functionality I get stuck.
Header data_location.hpp
class DataLocation
{
private:
public:
enum Params { model, period };
std::string getParamString(Params p);
};
Program data_location.cpp
string DataLocation::getParamString(Params p){
static const char * ParamsStrings[] = {"MODEL", "PERIOD"};
return ParamsStrings[p];
}
The array ParamsStrings should be generally available in the class, because I need a second method (with inverse function) returning the enum value given a string.
If I try to define the array in the header I get the error:
in-class initialization of static data member ‘const char* DataLocation::ParamsStrings []’ of incomplete type
Why is the type incomplete? The compiler is for sure able to counts the strings in the array, isn't it?
In case there is no way to get my code working, is there an other way? With 1) no XML; 2) no double definition of the strings; 3) not outside the class; 4) no in code programmed mapping.
In class (header) use keyword static and initialize it outside (.cpp) without the static keyword:
class DataLocation {
public:
enum Params { model, period };
string getParamString(Params p);
static const char* ParamsStrings[];
// ^^^^^^
};
const char* DataLocation::ParamsStrings[] = {"MODEL", "BLLBLA"};
//^^^^^^^^^^^^^^^^^^^^^^^^
The code you have posted is perfectly fine.
Here's the proof:
#include <iostream>
#include <string>
struct DataLocation
{
enum Params { model, period };
std::string getParamString(Params p){
static const char * ParamsStrings[] = {"MODEL", "PERIOD"};
return ParamsStrings[p];
}
};
int main()
{
auto a = DataLocation();
std::cout << a.getParamString(DataLocation::model) << std::endl;
return 0;
}
The error message you are getting is not to do with definition of a static data member in an inline function - that's allowed.
There's something else you're not showing us.
The main issue in my question (the second part) was that if I split the class in .hpp and .cpp the definition of the array (I mixed *char and string) has also to be split:
// data_location.hpp
class DataLocation {
static const char * ParamsStrings[];
}
// data_location.cpp
const char * ParamsStrings[] = {"MODEL", "PERIOD"};
At the end I introduced a consistency check to be sure that the number of values in enum growths as the number of strings. Because the array in C++ is somehow limited I had to go for a std::vector (to get the size).
Code for data_location.hpp
#ifndef DATA_LOCATION_HPP_
#define DATA_LOCATION_HPP_
#include <string>
#include "utils/dictionary.hpp"
extern const char* ENV_DATA_ROOT;
struct EDataLocationInconsistency : std::runtime_error
{
using std::runtime_error::runtime_error;
};
struct EDataLocationNotValidParam : std::runtime_error
{
using std::runtime_error::runtime_error;
};
class DataLocation
{
private:
std::string mRootLocation;
static const std::vector<std::string> msParamsStrings;
static bool msConsistenceCheckDone;
public:
DataLocation();
std::string getRootLocation();
std::string getLocation(Dictionary params);
enum Params { model, period, LAST_PARAM};
std::string Param2String(Params p);
Params String2Param(std::string p);
};
#endif
Code for data_location.cpp
#include "data_location.hpp"
#include <string>
#include <cstdlib>
using namespace std;
const char* ENV_DATA_ROOT = "DATA_ROOT";
bool DataLocation::msConsistenceCheckDone = false;
DataLocation::DataLocation() {
mRootLocation = std::getenv(ENV_DATA_ROOT);
if (not msConsistenceCheckDone) {
msConsistenceCheckDone = true;
if (LAST_PARAM+1 != msParamsStrings.size()) {
throw(EDataLocationInconsistency("DataLocation: Check Params and msParamsStrings"));
}
}
}
string DataLocation::getRootLocation() {
return mRootLocation;
}
string DataLocation::getLocation(Dictionary params) {
// to do
return "";
}
const vector<string> DataLocation::msParamsStrings = { "MODEL", "PERIOD", ""};
string DataLocation::Param2String(Params p) {
if (p>=msParamsStrings.size()) {
throw(EDataLocationNotValidParam("Parameter not found"));
}
return msParamsStrings[p];
}
DataLocation::Params DataLocation::String2Param(string p) {
for (int i = 0; i < msParamsStrings.size(); i++) {
if (p == msParamsStrings[i])
return (Params)i;
}
throw(EDataLocationNotValidParam("Parameter not found"));
}
And also a unit test:
#include <boost/test/unit_test.hpp>
#include "data_location.hpp"
#include <string>
using namespace std;
BOOST_AUTO_TEST_SUITE( data_location )
BOOST_AUTO_TEST_CASE(data_location_1) {
DataLocation dl;
auto s = dl.getRootLocation();
BOOST_CHECK_EQUAL(s, "/home/tc/data/forex" );
BOOST_CHECK_EQUAL(dl.Param2String(DataLocation::period),"PERIOD");
BOOST_CHECK_EQUAL(dl.String2Param("PERIOD"),DataLocation::period);
BOOST_CHECK_THROW(dl.String2Param("SOMETHING"), EDataLocationNotValidParam);
BOOST_CHECK_THROW(dl.Param2String((DataLocation::Params)100), EDataLocationNotValidParam);
}
BOOST_AUTO_TEST_SUITE_END()
C++ is very picky about what it will let you initialize inside of a class definition; there are some particularly non-intuitive rules surrounding static members. It all has to do with the ODR, and why all the rules are the way they are is not especially important.
To cut to the chase, making your array a static constexpr const member should shut the compiler up. With the C++11 standard, the restrictions were relaxed a bit, and one of the new stipulations was that static constexpr members can be initialized inline. This is perfect for your application, since the strings in your array are compile-time constants.
The recent g++ compiler which support C++0x or later compiles thus code. Pure C compile compiles, too. Because strings in initialization like {"MODEL", "PERIOD"}; implemented as const char * pointer to the char array.

How can I initialize char arrays in a constructor?

I'm having trouble declaring and initializing a char array. It always displays random characters. I created a smaller bit of code to show what I'm trying in my larger program:
class test
{
private:
char name[40];
int x;
public:
test();
void display()
{
std::cout<<name<<std::endl;
std::cin>>x;
}
};
test::test()
{
char name [] = "Standard";
}
int main()
{ test *test1 = new test;
test1->display();
}
And sorry if my formatting is bad, I can barely figure out this website let alone how to fix my code :(
If there are no particular reasons to not use std::string, do use std::string.
But if you really need to initialize that character array member, then:
#include <assert.h>
#include <iostream>
#include <string.h>
using namespace std;
class test
{
private:
char name[40];
int x;
public:
test();
void display() const
{
std::cout<<name<<std::endl;
}
};
test::test()
{
static char const nameData[] = "Standard";
assert( strlen( nameData ) < sizeof( name ) );
strcpy( name, nameData );
}
int main()
{
test().display();
}
Your constructor is not setting the member variable name, it's declaring a local variable. Once the local variable goes out of scope at the end of the constructor, it disappears. Meanwhile the member variable still isn't initialized and is filled with random garbage.
If you're going to use old-fashioned character arrays you'll also need to use an old-fashioned function like strcpy to copy into the member variable. If all you want to do is set it to an empty string you can initialize it with name[0] = 0.
Since you are using C++, I suggest using strings instead of char arrays. Otherwise you'd need to employ strcpy (or friends).
Also, you forgot to delete the test1 instance.
#include <iostream>
#include <string>
class test
{
private:
std::string name;
int x;
public:
test();
void display()
{
std::cout<<name<<std::endl;
}
};
test::test()
{
name = "Standard";
}
int main()
{
test test1;
test1.display();
std::cin>>x;
}
Considering you tagged the question as C++, you should use std::string:
#include <string>
class test
{
private:
std::string name;
int x;
public:
test();
void display()
{
std::cout<<name<<std::endl;
std::cin>>x;
}
};
test::test() : name("Standard")
{
}
c++11 actually provides two ways of doing this. You can default the member on it's declaration line or you can use the constructor initialization list.
Example of declaration line initialization:
class test1 {
char name[40] = "Standard";
public:
void display() { cout << name << endl; }
};
Example of constructor initialization:
class test2 {
char name[40];
public:
test2() : name("Standard") {};
void display() { cout << name << endl; }
};
You can see a live example of both of these here: http://ideone.com/zC8We9
My personal preference is to use the declaration line initialization because:
Where no other variables must be constructed this allows the generated default constructor to be used
Where multiple constructors are required this allows the variable to be initialized in only one place rather than in all the constructor initialization lists
Having said all this, using a char[] may be considered damaging as the generated default assignment operator, and copy/move constructors won't work. This can be solved by:
Making the member const
Using a char* (this won't work if the member will hold anything but a literal string)
In the general case std::string should be preferred