The differences between if else and #if #else #endif

Question

I am confused between the if/else and #if/#else/#endif constructs.

1. What are the differences between them?
2. In which specific situations should I use each of them?

Answer 1

I am confused about the if/else and #if/#else/#endif. It seems that they have the same logic functionality.

Can I ask what's the differences between them?

#if, #else and #endif belong to preprocessing. They are not executed but are instructions for textual replacement. You can think of them as a kind of automatic "search & replace" feature you'd usually find in a text editor.

if and else are run-time constructs. You can think of them as being executed while the program runs.

Let's say you have this program:

#include <iostream>

#define VALUE 1

int main()
{
#if VALUE == 1
    std::cout << "one\n";
#else
    std::cout << "not one\n";
#endif
}

When you tell your compiler to compile this program, the preprocessor will make a textual replacement before the "real" C++ code is actually compiled. It will be as if the program was:

#include <iostream>

int main()
{
    std::cout << "one\n";
}

Now, technically you could use an if here:

#include <iostream>

#define VALUE 1

int main()
{
    if (VALUE == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

But in C++ you don't use #define for constants. You'd instead have something like:

#include <iostream>

int const value = 1;

int main()
{
    if (value == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

Perhaps the value is only known while the program executes, e.g. via user input. Then you obviously cannot use #if, which only works before the program runs. You must use if:

#include <iostream>

int main()
{
    int value;
    std::cin >> value; // gross simplification here

    if (value == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

What kind of specific situations for me to choose each of them?

A good guideline for a beginner would be: Use #if (or actually: #ifndef) only for include guards. Consider further uses of #if when you encounter problems that can only be solved by the preprocessor.

Answer 2

if(...) and else(...) conditions are evaluated at runtime. #if, #else are evaluated before compile time by the preprocessor.

Answer 3

Can I ask what's the differences between them?

#if #else and #endif are instructions to the compiler, to only compile the code between them, if a compilation level condition (like a macro being defined or having a certain value) is satisfied.

if and else are parts of the compiled algorithm.

What kind of specific situations for me to choose each of them?

The pre-compilation conditions are used to disable parts of the code, in situations where they make no sense (like calls to Windows-specific APIs, when compiling under Linux). They are key to developing cross-platform code (for example).