c++ - error C2106: '=' : left operand must be l-value

Question

Looking at the other questions regarding error C2106, I am still lost as to what the issue is with my code. While compiling I get the following errors:

c:driver.cpp(99): error C2106: '=' : left operand must be l-value

c:driver.cpp(169): error C2106: '=' : left operand must be l-value

The line of code is as follows:

payroll.at(i) = NULL; //Line 99
payroll.at(count++) = ePtr; //Line 169

I am failing to understand why this error is being thrown. In this project I have changed my driver.cpp from an array of employee object pointers to a custom Vector template that I made. I declare the Vector as follows...

//Declare an Vector to hold employee object pointers
MyVector <employee*> payroll;

Any help is appreciated...

Answer 1

This error is being thrown for the same reason you can't do something like this:

36 = 3;

Your version of Vector::at should be returning a reference rather than a value.
Lvalues are called Lvalues because they can appear on the left of an assignment. Rvalues cannot appear on the left side, which is why we call them rvalues. You can't assign 3 to 36 because 36 is not an lvalue, it is an rvalue, a temporary. It doesn't have a memory address. For the same reason, you cannot assign NULL to payroll.at(i).

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Your definition:

template <class V> V MyVector<V>::at(int n)

What it should be:

template<typename V> V& MyVector::at(std::size_t n)
template<typename V> const V& MyVector::at(std::size_t n) const