Get to Know the New C++11 Initialization Forms
Like this article? We recommend
Like this article? We recommend
Like this article? We recommend
Initialization in C++03 is tricky, to say the least, with four different initialization notations and far too many arbitrary restrictions and loopholes:
- No way to initialize a member array
- No convenient form of initializing containers
- No way to initialize dynamically allocated POD types
C++11 fixes these problems with new facilities for initializing objects. In this article, I present the new C++11 brace-initialization notation, discuss class member initialization, and explain how to initialize containers by using initializer lists.
C++03 Initialization: A Tale of Four Initialization Notations
To appreciate the new initialization syntax of C++11, let's look at the C++03 initialization Babel first. C++03 has various categories of initialization:
- Initialization of fundamental types. The initialization of fundamental types uses the equal sign (=):
int n=0; void*p=0; char c='A';
//C++03 initialization of classes and objects
struct S1
{
explicit S1(int n, int m) : x(n), y(m){} //mem-init
private:
int x, y;
};
S1 s(0,1); //object initializers enclosed in parentheses
S1 s2={0,1}; //compilation error
//C++03: POD arrays and structs are aggregates
int c1[2]={0,2};
char c2[]="message";
//or you can use the more verbose form:
char c3[]={'m','e','s','s','a','g','e','\0'};
struct S
{
int a,b;
};
S s={0,1};
You can use parentheses to initialize fundamental types as well. The parentheses in this case are interchangeable with the equal sign notation:
int n(0); //same as int n=0; double d(0.5);
C++03 Initialization: Arbitrary Restrictions and Loopholes
To add insult to injury, C++03 imposes arbitrary restrictions in some cases, such as the inability to initialize member arrays:
class C
{
int x[100];
C(); //no proper way to initialize x
};
Similarly, you can't initialize a dynamically allocated POD array:
char *buff=new char[1024]; //no proper way to initialize the elements of buff
Finally, there's no easy way to initialize the elements of a Standard Library container. For instance, if you want to initialize a vector of strings, you'd normally use a sequence of push_back() calls like this:
vector <string> vs;
vs.push_back("alpha");
vs.push_back("beta");
vs.push_back("gamma");
To conclude, C++03 initialization is a mess. Let's see how C++11 tackles these problems with its new and uniform initialization notation.
Introducing C++11 Brace-Initialization
C++11 attempts to overcome the problems of C++03 initialization by introducing a universal initialization notation that applies to every type—whether a POD variable, a class object with a user-defined constructor, a POD array, a dynamically allocated array, or even a Standard Library container. The universal initializer is called a brace-init. It looks like this:
//C++11 brace-init
int a{0};
string s{"hello"};
string s2{s}; //copy construction
vector <string> vs{"alpha", "beta", "gamma"};
map<string, string> stars
{ {"Superman", "+1 (212) 545-7890"},
{"Batman", "+1 (212) 545-0987"}};
double *pd= new double [3] {0.5, 1.2, 12.99};
class C
{
int x[4];
public:
C(): x{0,1,2,3} {}
};
Notice that unlike the traditional aggregate initializer of C and C++03, which uses braces after an equal sign (={}), the C++11 brace-init consists of a pair of braces (without the equal sign) in which the initializer(s) will be enclosed. An empty pair of braces indicates default initialization. Default initialization of POD types usually means initialization to binary zeros, whereas for non-POD types default initialization means default construction:
//C++11: default initialization using {}
int n{}; //zero initialization: n is initialized to 0
int *p{}; //initialized to nullptr
double d{}; //initialized to 0.0
char s[12]{}; //all 12 chars are initialized to '\0'
string s{}; //same as: string s;
char *p=new char [5]{}; // all five chars are initialized to '\0'
Class Member Initialization
C++11 pulls another rabbit out of its hat with class member initializers. Perhaps an example will best illustrate these:
class C
{
int x=7; //class member initializer
public:
C();
};
The data member x is automatically initialized to 7 in every instance of class C. In former dialects of C++, you would use the more cumbersome mem-init notation for the same purpose:
class C
{
int x;
public:
C() : x(7) {}
};
C++11 class member initializers are mostly a matter of convenience. They provide an overt and simplified form of initializing data members. But class member initializers also let you perform a few tricks that have hitherto been impossible. For example, you can use a class member initializer to initialize a member array:
class C
{
int y[5] {1,2,3,4};
public:
C();
};
Notice that a class member initializer can consist of any valid initialization expression, whether that's the traditional equal sign, a pair of parentheses, or the new brace-init:
class C
{
string s("abc");
double d=0;
char * p {nullptr};
int y[5] {1,2,3,4};
public:
C();
};
Regardless of the initialization form used, the compiler conceptually transforms every class member initializer into a corresponding mem-init. Thus, class C above is semantically equivalent to the following class:
class C2
{
string s;
double d;
char * p;
int y[5];
public:
C() : s("abc"), d(0.0), p(nullptr), y{1,2,3,4} {}
};
Bear in mind that if the same data member has both a class member initializer and a mem-init in the constructor, the latter takes precedence. In fact, you can take advantage of this behavior by specifying a default value for a member in the form of a class member initializer that will be used if the constructor doesn't have an explicit mem-init for that member. Otherwise, the constructor's mem-init will take effect, overriding the class member initializer. This technique is useful in classes that have multiple constructors:
class C
{
int x=7; //class member initializer
C(); //x is initialized to 7 when the default ctor is invoked
C(int y) : x(y) {} //overrides the class member initializer
};
C c; //c.x = 7
C c2(5); //c.x = 5
Initializer Lists and Sequence Constructors
An initializer list lets you use a sequence of values wherever an initializer can appear. For example, you can initialize a vector in C++11 like this:
vector<int> vi {1,2,3,4,5,6};
vector<double> vd {0.5, 1.33, 2.66};
You may include as many initializers as you like between the braces. Although superficially this new syntax seems identical to the brace-init notation we discussed earlier, behind the scenes it's a different story. C++11 furnishes every STL container with a new constructor type called a sequence constructor. A sequence constructor intercepts initializers in the form of {x,y...}. To make this machinery work, C++11 introduced another secret ingredient: an auxiliary class template called std::initializer_list<T>. When the compiler sees an initializer list, say {0.5, 1.33, 2.66}, it transforms the values in that list into an array of T with n elements (n is the number of values enclosed in braces) and uses that array to populate an implicitly generated initializer_list<T> object. The class template initializer_list has three member functions that access the array:
template<class E> class initializer_list
{
//implementation (a pair of pointers or a pointer + length)
public:
constexpr initializer_list(const E*, const E*); // [first,last)
constexpr initializer_list(const E*, int); // [first, first+length)
constexpr int size() const; // no. of elements
constexpr const T* begin() const; // first element
constexpr const T* end() const; // one more than the last element
};
To better understand how the compiler handles initializer lists of containers, let's dissect a concrete example. Suppose your code contains the following declaration of a vector:
vector<double> vd {0.5, 1.33, 2.66};
The compiler detects the initializer list {0.5, 1.33, 2.66} and performs the following steps:
- Detect the type of the values in the initializer list. In the case of {0.5, 1.33, 2.66}, the type is double.
- Copy the values from the list into an array of three doubles.
- Construct an initializer_list<double> object that "wraps" the array created in the preceding step.
- Pass the initializer_list<double> object by reference to vd's sequence constructor. The constructor in turn allocates n elements in the vector object, initializing them with the values of the array.
It's hard to imagine that so much is going on behind the scenes every time you initialize an STL container with a pair of braces! The good news is that you don't have to do anything for this magic to happen—it just works. Of course, you still need a C++11-compliant compiler as well as a C++11-compliant Standard Library to use initializer lists. Make sure that your target project is built with the appropriate compilation options, too.
In Conclusion
The C++ standards committee invested a lot of time and effort in finding a solution to the limitations of C++03 initialization. It looks like they succeeded. Historically, brace-init, class member initializers, and initializer lists were three independent proposals. Later, they were revised to ensure compatibility and uniformity. Together, these three initialization-related features make C++11 programming simpler and more intuitive. You will surely appreciate them next time you initialize a dynamically allocated array—or, indeed, a vector.
Danny Kalev is a certified system analyst and software engineer specializing in C, C++ Objective-C and other programming languages. He was a member of the C++ standards committee until 2000 and has since been involved informally in the C++ standardization process. Danny is the author of ANSI/ISO Professional Programmer's Handbook (1999) and The Informit C++ Reference Guide: Techniques, Insight, and Practical Advice on C++ (2005). He was also the Informit C++ Reference Guide. Danny earned an M.A. in linguistics, graduating summa cum laude, and is now pursuing his Ph.D. in applied linguistics.
You might also like: