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Distinguish the differences between R-values and L-values in C++

Learn to understand the distinction between an r-value and an l-value in C++ to avoid common compilation errors.

By Danny Kalev

The programming language C++ inherited the convention that allows for distinction between an r-value and an l-value. Understanding what these concepts mean will help you avoid common pitfalls and decipher cryptic compilation errors.

The definition of values

An object is basically a region of memory storage. An l-value is an expression that refers to such an object. The original definition of an l-value was "an object that can appear on the left-hand side of an assignment". However, this historical definition isn't accurate because const objects, which are l-values, cannot appear on the left-hand side of an assignment.

An expression that can appear only on the right-hand side of an expression is an r-value. The following example contains instances of l-values and r-values:

#include <string>
using namespace std;
int& f();

void func()
{
  int n;
  char buf[2]; //buf is an r-value
  buf[0]= 'a';  // 'buf[0]' is an l-value, 'a' is an r-value
  n=7;   // n is an l-value; the literal 7 is an r-value
  string s1="a", s2="b", s3="c"; // "a", "b", "c" are r-values
  s1=string("a");  // temporaries are r-values
  int* p=new int; // p is an l-value; 'new int' is an r-value
  f()=9; //a function call returning a reference is an l-value
  s1.size(); //otherwise, a function call is an r-value
}

An l-value can appear in a context that requires an r-value. In this case, the l-value is implicitly converted to an r-value. However, you cannot place an r-value in a context that requires an l-value.

Practical application

The distinction between an l-value and an r-value can be used for eliminating a common bug. Programmers who mistakenly replace the = operator for the == operator can reverse the order of an equality expression, placing the r-value on the left, like this:

if (0==x) //instead of x==0

When placing the r-value on the left, mistyping the == operator as the = operator triggers a compilation error:

if (0=x) // error: "L-value required"

Although this isn't the most intelligible error message, it certainly catches the bug.

Danny Kalev is a systems analyst and software engineer with 14 years of experience, specializing in C++ and object-oriented design.

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