2d barcode generator vb.net int a[8]; a[5] = 22; // a[5] is a mutable lvalue int* p = &n; *p = 77; // *p is a mutable lvalue in Software

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int a[8]; a[5] = 22; // a[5] is a mutable lvalue int* p = &n; *p = 77; // *p is a mutable lvalue
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Other examples of immutable lvalues include arrays, functions, and references. In general, an lvalue is anything whose address is accessible. Since an address is what a reference variable needs when it is declared, the C++ syntax requirement for such a declaration specifies an lvalue:
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type& refname = lvalue;
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For example, this is a legal declaration of a reference:
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int& r = n; // // // // OK: n is an lvalue ERROR: 44 is not an lvalue ERROR: n++ is not an lvalue ERROR: cube(n) is not an lvalue
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int& r = 44; int& r = n++; int& r = cube(n);
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7.7 RETURNING A REFERENCE A function s return type may be a reference provided that the value returned is an lvalue which is not local to the function. This restriction means that the returned value is actually a reference to an lvalue that exists after the function terminates. Consequently that returned lvalue may be used like any other lvalue; for example, on the left side of an assignment: EXAMPLE 7.8 Returning a Reference
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int& max(int& m, int& n) { return (m > n m : n); } // return type is reference to int // m and n are non-local references
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CHAP. 7]
POINTERS AND REFERENCES
int main() { int m = 44, n = 22; cout << m << ", " << n << ", " << max(m,n) << endl; max(m,n) = 55; // changes the value of m from 44 to 55 cout << m << ", " << n << ", " << max(m,n) << endl; } 44, 22, 44 55, 22, 55
The max() function returns a reference to the larger of the two variables passed to it. Since the return value is a reference, the expression max(m,n) acts like a reference to m (since m is larger than n). So assigning 55 to the expression max(m,n) is equivalent to assigning it to m itself.
EXAMPLE 7.9 Using a Function as an Array Subscript
float& component(float* v, int k) { return v[k-1]; } int main() { float v[4]; for (int k = 1; k <= 4; k++) component(v,k) = 1.0/k; for (int i = 0; i < 4; i++) cout << "v[" << i << "] = " << v[i] << endl; } v[0] = 1 v[1] = 0.5 v[2] = 0.333333 v[3] = 0.25
The component() function allows vectors to be accessed using the scientific 1-based indexing instead of the default 0-based indexing. So the assignment component(v,k) = 1.0/k is really the assignment v[k+1] = 1.0/k . We ll see a better way to do this in 10.
7.8 ARRAYS AND POINTERS Although pointer types are not integer types, some integer arithmetic operators can be applied to pointers. The affect of this arithmetic is to cause the pointer to point to another memory location. The actual change in address depends upon the size of the fundamental type to which the pointer points. Pointers can be incremented and decremented like integers. However, the increase or decrease in the pointer s value is equal to the size of the object to which it points: EXAMPLE 7.10 Traversing an Array with a Pointer
This example shows how a pointer can be used to traverse an array. int main() { const int SIZE = 3; short a[SIZE] = {22, 33, 44};
POINTERS AND REFERENCES
[CHAP. 7
cout << "a = " << a << endl; cout << "sizeof(short) = " << sizeof(short) << endl; short* end = a + SIZE; // converts SIZE to offset 6 short sum = 0; for (short* p = a; p < end; p++) { sum += *p; cout << "\t p = " << p; cout << "\t *p = " << *p; cout << "\t sum = " << sum << endl; } cout << "end = " << end << endl; } a = 0x3fffd1a sizeof(short) = 2 p = 0x3fffd1a *p = 22 sum = 22 p = 0x3fffd1c *p = 33 sum = 55 p = 0x3fffd1e *p = 44 sum = 99 end = 0x3fffd20 The second line of output shows that on this machine short integers occupy 2 bytes. Since p is a pointer to short, each time it is incremented it advances 2 bytes to the next short integer in the array. That way, sum += *p accumulates their sum of the integers. If p were a pointer to double and sizeof(double) were 8 bytes, then each time p is incremented it would advance 8 bytes.
Example 7.10 shows that when a pointer is incremented, its value is increased by the number SIZE (in bytes) of the object to which it points. For example,
float a[8]; float* p = a; // p points to a[0] ++p; // increases the value of p by sizeof(float) If floats occupy 4 bytes, then ++p; increases the value of p by 4, and p += 5; increases the value of p by 20. This is how an array can be traversed: by initializing a pointer to the first
element of the array and then repeatedly incrementing the pointer. Each increment moves the pointer to the next element of the array. We can also use a pointer for direct access into the array. For example, we can access a[5] by initializing the pointer to a[0] and then adding 5 to it:
float* p = a; p += 5; // // p points to a[0] now p points to a[5]
So once the pointer is initialized to the starting address of the array, it works like an index. Warning: In C++ it is possible to access and even modify unallocated memory locations. This is risky and should generally be avoided. For example,
float a[8]; float* p = a[7]; ++p; *p = 22.2; // p points to last element in the array // now p points to memory past last element! // TROUBLE!
The next example shows an even tighter connection between arrays and pointers: the name of an array itself is a const pointer to the first element of the array. It also shows that pointers can be compared.
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