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Demotion from a longer integral type to a shorter integral type (such as long to int) results in discarding the leftmost (most significant) bits in the binary number representation. The result may be a drastically different number. |
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Demotion from a floating point type to an integral type causes truncation of the fractional part (and an undefined result if the whole-number part will not fit into the destination variable). The result of truncating a negative number is machine-dependent. |
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Demotion from a longer floating point type to a shorter floating point type (such as double to float) may result in a loss of digits of precision. |
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Our description of type coercion in an assignment operation also holds for parameter passage (the mapping of actual parameters onto formal parameters) and for returning a function value with a return statement. For example, assume that INT_MAX on your machine is 32767 and that you have the following function: |
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void DoSomething( int n )
{
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} |
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If the function is called with the statement |
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then the value 50000 (which is implicitly of type long because it is larger than INT_MAX) is demoted to a completely different, smaller value that fits into an int location. In a similar fashion, execution of the function |
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int SomeFunc( float x )
{
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return 70000;
} |
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causes demotion of the value 70000 to a smaller int value because int is the declared type of the function return value. |
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One interesting consequence of implicit type coercion is the futility of declaring a variable to be unsigned, hoping that the compiler will prevent you from making a mistake like this: |
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