Help requested with a dynamic array.

[brutaltomrammen]

SO I'm very new to c++ (as in a few weeks) and I've been reading the C++ Primer Plus by Stephen Prata. At the end of each chapter he outlines programming exercises. The one I'm stuck on involves using new to create a dynamic array. Here's the code

#include <iostream>
#include <string>

void tag(int);
using namespace std;

struct candybar
{
	char name[20];
	double weight;
	int calories;
};

int main()
{
	tag(0);
	tag(1);
	tag(2);
	return 0;
}

void tag(int n)
{
		
    candybar *pc = new candybar [3];
    pc[0] = {"Mars" , 2.75, 245};   //Error on this line
    pc[1] = {"Butterfinger", 2.85, 300};  //Error on this line
    pc[2] = {"Twix", 2.12, 150};  //Error on this line
	cout << "The Candy Company is releasing a new treat called " << pc[n].name << " which weighs a total of " << pc[n].weight ;
	cout <<" pounds and only contains " << pc[n].calories << " calories.\n";
};

I'm using Visual Studio Express 2012 and I'm getting the build error "Error C2059: syntax error : '{'" and intellisense is telling me it "expected an expression." I could use some help understanding this, thanks in advance.

[Peter87]

You can only use the {}-syntax to initialize the object when the object is created. When creating an array with new candybar [3] the objects in the array will be default initialized, and in C++03 there is no way around this.

Instead of
pc[0] = {"Mars" , 2.75, 245};
do would have to do something like:

std::strcpy(pc[0].name, "Mars");
pc[0].weight = 2.75;
pc[0].calories = 245;

In C++11 you can initialize the objects in the array when the array is created like this:

candybar *pc = new candybar [3] 
{
	{"Mars" , 2.75, 245}, 
	{"Butterfinger", 2.85, 300}, 
	{"Twix", 2.12, 150}
};

You can also do it the way you did it if you add candybar after the = symbols.

pc[0] = candybar{"Mars" , 2.75, 245};
pc[1] = candybar{"Butterfinger", 2.85, 300};
pc[2] = candybar{"Twix", 2.12, 150};

[JLBorges]

Your code is fine, it should compile without errors. http://liveworkspace.org/code/5waqH$0

Requires compiler support for uniform initialization http://www.stroustrup.com/C++11FAQ.html#uniform-init

The November 2012 CTP would have it. http://www.microsoft.com/en-us/download/details.aspx?id=35515


You need to delete[] the array before you return from tag(), or else there would be a leak.
Better still, just use a std::vector<> instead http://www.mochima.com/tutorials/vectors.html

[brutaltomrammen]

@JLBorges I tried installing the ctp and the problem persists.

[JLBorges]

Have you changed the project configuration to switch to the new compiler?
See the section on 'Installation and Usage' http://blogs.msdn.com/b/vcblog/archive/2012/11/02/visual-c-c-11-and-the-future-of-c.aspx

Note: I don't have a Windows machine at hand; so I can't test it with the Microsoft compiler.

[brutaltomrammen]

Yes, I went through all of the steps of the installation.

[cire]

The November 2012 CTP seems to have some issues with uniform initialization.

In addition to the code in the OP not being accepted, one cannot specify the type explicitly pc[0] = candybar{"Mars", 2.75, 245};.

[JLBorges]

Thanks, brutaltomrammen and cire.

This should compile cleanly:

#include <iostream>
#include <string>
#include <vector> // *** added

void tag(int);
using namespace std;

struct candybar
{
	std::string name; // *** changed
	double weight;
	int calories;
};

int main()
{
	tag(0);
	tag(1);
	tag(2);
}

void tag(int n)
{

    //candybar *pc = new candybar [3];
    std::vector<candybar> pc(3) ; // dynamic array of 3 candybars

    candybar a = {"Mars" , 2.75, 245}; pc.push_back(a) ;
    candybar b = {"Butterfinger", 2.85, 300}; pc.push_back(b) ;
    candybar c = {"Twix", 2.12, 150}; pc.push_back(c) ;

    cout << "The Candy Company is releasing a new treat called " << pc[n].name 
         << " which weighs a total of " << pc[n].weight ;
    cout <<" pounds and only contains " << pc[n].calories << " calories.\n";
}

void tag_1(int n) // or
{

    candybar *pc = new candybar [3];

    candybar a = {"Mars" , 2.75, 245}; pc[0] = a ;
    candybar b = {"Butterfinger", 2.85, 300}; pc[1] = b ;
    candybar c = {"Twix", 2.12, 150}; pc[2] = c ;

    cout << "The Candy Company is releasing a new treat called " << pc[n].name 
         << " which weighs a total of " << pc[n].weight ;
    cout <<" pounds and only contains " << pc[n].calories << " calories.\n";

    delete[] pc ;
}