pi = ival; // error in Java

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pi = ival; // error
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(The set of implicit type conversions recognized by the language is discussed in Section 4 14) The left operand of the assignment operator must be an lvalue that is, it must have an associated writable address value An obvious example of an illegal non-lvalue assignment is the following:
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1024 = ival; // error
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Here is one possible solution:
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int value = 1024; value = ival; // ok
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However, in some cases, having an lvalue in itself is not sufficient For example, given the following object definitions
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const int array_size = 8; int ia[ array_size ] = { 0, 1, 2, 2, 3, 5, 8, 13 }; int *pia = ia;
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the assignment
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array_size = 512; // error
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm
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is illegal even though array_size is an lvalue; the const definition of array_size makes its address value not writable Similarly, the assignment
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ia = pia; // error
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is illegal Although ia is an lvalue, an array object itself cannot be assigned to; only the elements it contains can be assigned to The assignment
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pia + 2 = 1; // error
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is also illegal Although pia+2 yields the address of ia[2], the result is not a writable address value However, if the dereference operator is applied to the address value, as in
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*(pia + 2) = 1; // ok
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the assignment is ok: the dereference operator indicates that the assignment is to the object to which pia+2 refers The result of an assignment is the value actually placed in the left operand's associated storage For example, the result of
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ival = 0;
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is 0, whereas the result of
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ival = 314159;
is 3, both of type int Because of this, an assignment can appear as a subexpression For example, the following while loop
extern char next_char(); int main() { char ch = next_char(); while ( ch != '\n' ) { // do something ch = next_char(); } // }
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm
can be rewritten as follows:
extern char next_char(); int main() { char ch; while (( ch = next_char() ) != '\n' ) { // do something } // }
The additional parentheses are necessary because of the lower precedence of the assignment operator to that of inequality Precedence determines the order of evaluation within an expression, with higher operators being evaluated first Without parentheses, the inequality test
next_char() != '\n'
is evaluated first, and ch is then assigned the true or false value of whether the result of next_char() is equal to the newline character clearly not what we intended! (We look at precedence in detail in Section 413) Similarly, assignment operators can be concatenated provided that each of the operands being assigned is of the same general data type For example, in the following
int main() { int ival, jval; ival = jval = 0; // ok: each assigned 0 // }
ival and jval are each assigned 0 However, the following is illegal, because ival and pval are objects of different types, although 0 is a valid value that can be assigned to either:
int main() { int ival; int *pval; ival = pval = 0; // error: not the same types // }
The following may or may not be legal; however, it does not serve as a definition of both ival and jval:
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file:///F|/WinDDK/resources/CPPPrimer/c++primerhtm
int main() { // int ival = jval = 0; // may or may not be legal // }
The example is legal only if jval has been previously defined and is of the appropriate type to be assigned zero In that case, ival is initialized to the result of assigning jval a value of 0, which itself is 0 In order for it to define both objects, it must be rewritten as follows:
int main() { // ok: defines ival and jval int ival = 0, jval = 0; // }