Unary Plus Operator in C : What Does It Do and Why Does It Matter?

C unary plus operator: functions and usage

At first glance, the unary plus operator seems redundant and mysterious. However, it plays a vital role in programming languages, especially C.

The function of unary plus operator

Contrary to its name, the unary plus operator does have a significant impact. It applies "usual arithmetic conversions" to the operands. This means it converts the operand to a new value, possibly changing its data type.

Convert to integer type

For integers, the unary plus operator converts the operand to a larger-width signed integer. For example, if the operand is an unsigned short, it is converted to a signed integer of type 'int'.

C Example

Consider the following C code:

<code class="language-c++">int main() {
    unsigned short x = 5;
    std::cout << +x << std::endl; // 输出 5
    return 0;
}</code>

When the unary plus operator is applied to x, it creates a new value that is a signed integer of type 'int'.

Impact of type conversion

This type conversion may have practical implications. For example, consider a function foo that accepts an unsigned short argument:

<code class="language-c++">void foo(unsigned short x) {
    std::cout << "x is an unsigned short" << std::endl;
}</code>

If you call this function with x as argument, it behaves as if you passed an 'int' to the function:

<code class="language-c++">unsigned short x = 5;
foo(+x);  // 输出 "x is an unsigned short" (非预期结果)</code>

Therefore, it is important to avoid using the unary plus sign as a simple annotation to indicate that an integer is positive, as it may cause unexpected behavior due to type casting.

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