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Your "getters" should be const since they don't modify any data members. This will also protect you against any accidental modifications in those functions.

 unsigned int getLength() const { return length; }

The constructor can instead be an initializer list:

 linked_list() : length(0), start(NULL) {}

Be aware of the issues regarding using namespace std.

If you have a C++11-compliant compiler, use nullptr instead of NULL.

Be aware that _tmain() and _TCHAR* are non-portable, so anyone using this code outside of Visual Studio will not be able to compile it. In order to make this portable, change them to main() and char* respectively.

Since you're using std::pow(), you should include <cmath>. Although <iostream> is already covering it, you should add this library anyway.

There's not really a need for getLength2(). You just need to increment or decrement length, depending on the list operation. Your inline "getter" will, of course, return this updated length.

Regarding incrementing/decrementing, length += 1 and length -= 1 can respectively be length++ and length--.

The val argument for add() should be passed by const&, especially if it happens to be of a non-primitive type.

You should consider error-checking (empty list) in pop() if you'd still like it to return the popped value. Otherwise, you can make it void and simply return if the list is already empty.

Instead of (*start).value, use the -> operator for readability: start->value.

Your "getters" should be const since they don't modify any data members. This will also protect you against any accidental modifications in those functions.

 unsigned int getLength() const { return length; }

The constructor can instead be an initializer list:

 linked_list() : length(0), start(NULL) {}

Be aware of the issues regarding using namespace std.

If you have a C++11-compliant compiler, use nullptr instead of NULL.

Be aware that _tmain() and _TCHAR* are non-portable, so anyone using this code outside of Visual Studio will not be able to compile it. In order to make this portable, change them to main() and char* respectively.

Since you're using std::pow(), you should include <cmath>. Although <iostream> is already covering it, you should add this library anyway.

There's not really a need for getLength2(). You just need to increment or decrement length, depending on the list operation. Your inline "getter" will, of course, return this updated length.

Regarding incrementing/decrementing, length += 1 and length -= 1 can respectively be length++ and length--.

The val argument for add() should be passed by const&, especially if it happens to be of a non-primitive type.

You should consider error-checking (empty list) in pop() if you'd still like it to return the popped value. Otherwise, you can make it void and simply return if the list is already empty.

Instead of (*start).value, use the -> operator for readability: start->value.