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Traps

CS253: C++ traps for the unwary Java programmer

This is not a list of all Java/C++ differences. Instead, it is a list of differences that are likely to cause problems for a programmer who knows Java but is learning C++.

Age

• C++: 1983
• Java: 1995

If you ask, “Why doesn’t C++ do such-and-such a thing like Java?”, then you’re asking the wrong question.

Methods & Functions

• Java: methods
• C++: functions & methods

In Java, everything’s a method, because everything’s in a class.

C++ has methods inside of classes. However, C++ also has functions outside of classes, like main().

main

• Java: void main(String[] args)
• C++: int main()
• C++: int main(int argc, char *argv[])

In C++, main returns an int indicating success/failure. Zero indicates success, positive numbers indicate failure. It’s not a boolean success indicator. It’s an integer code.

In C++, main uses an array of old-style C char * strings, for compatibility with C.

Poor George Boole

• Java: boolean
• C++: bool

Byte

• Java: char, byte
• C++: char

C++’s char type is the closest approximation to Java’s byte.

Java’s char can hold Unicode characters. C++’s char has an implementation-defined size, but it’s typically a single byte.

final/const/constexpr

• Java: final
• C++: final, const, constexpr

In Java, final indicates a non-overrideable method, or a constant value.

In C++, final indicates a non-overridable method. const indicates a method that doesn’t alter object state, or a value that you can’t change. constexpr indicates a compile-time constant.

Arrays

• Java: int[] a = new int[100];
• C++: int a[100];

In C++, arrays are not objects. Hence, they have no methods. Hence, you can’t ask an array how long it is. Use a vector or std::array instead, if you want that.

The simple term “array” is, alas, ambiguous. We will use the phrases “C-style array” or std::array to resolve this.

Strings

• Java: String
• C++: string

In Java, classes start with a capital letter. That’s not always so in C++.

Objects: heap or not?

• Java: String s = new String;
• C++: string s;

In Java, objects are always in the heap—dynamic memory. In C++, they can be, but don’t have to be.

String Subscripting

• Java: char c = s.charAt(3);
• C++: char c = s[3];

Sure, C++’s string class has an .at() method, but, why?

Order of Operations

• Java: int n=0; int v = ++n + ++n;
• C++: int n=0; int v = ++n + ++n;

The Java example yields 3, because the order of operations is defined by the language.

The C++ example invokes undefined behavior. The compiler is free to do those operations (++, ++, +) in whatever order it considers to be best.

Plus-sign overloading

• Java: "foobar"+3
• C++: "foobar"+3

In Java, + is overloaded to handle arguments of String and int, and so yields "foobar3".

C++ performs address arithmetic in this case, and so yields "bar".

Pointers and references

• Java: references
• C++: pointers, references

Java has pointers, which it calls “references”.

C++ has explicit pointers, and references like Java’s.

Method/Member Access

• Java: alpha.beta()
• C++: alpha.beta() or alpha->beta()

In Java, alpha can only be a reference.

In C++, alpha might be an object, a reference to an object, or a pointer to an object.

Garbage Collection

• Java: garbage collection
• C++: delete

Copying

• Java: obj1 = obj2;
• C++: obj1 = obj2;

In Java, this copies a reference. No new object is created, no real data is copied.

In C++, the copy ctor of the class is called. This typically copies the data in the object.

Out of Bounds

• Java: a[-12]
• C++: a[-12]

Java throws an exception.

C++ assumes that you know what you’re doing.

Nothingness

• Java: null
• C++: nullptr or NULL or 0

In Java, null is the reference to nothing.

In C++, a pointer that points to nothing useful can be initialized to nullptr or NULL or 0.

• nullptr is best
• NULL is compatible with C
• 0 is just plain stupid

The Biggest Difference

• C++: rules
• Java: drools