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While solving the problem TTM on SPOJ using a Persistent Segment Tree, I ran into a very frustrating bug

My code was logically correct, yet I was getting completely different verdicts depending on the compiler:

• Accepted on C++14 (Clang)
• Wrong Answer on C++14 (GCC)

After a long debugging, the issue turned out to be caused by unspecified evaluation order

Code

Here is the problematic line:

// 't' is the current version number of the Persistent Segment Tree
if (op == 'C') {
    int l, r, d;
    cin >> l >> r >> d;
    --l, --r;

    // The fatal line:
    pst.roots[++t] = pst.update(t, l, r, d);
}

What’s the Problem?

In C++14 and older, the C++ standard does not specify the evaluation order of the operands of the assignment operator (=)

That means the compiler is free to evaluate either:

• the left-hand side first (pst.roots[++t])
• or the right-hand side first (pst.update(t, ...))

As a result, different compilers may execute the statement differently.

How Clang Evaluated It (Right-to-Left) → AC

Clang evaluated the right-hand side first:

pst.update(t, ...)

At this moment, t still had its old value (say 0).

So the update correctly created a new version based on:

roots[0]

Then Clang evaluated:

++t

which incremented t to 1, and finally assigned the result to:

roots[1]

This happened to match my intended logic.

How GCC Evaluated It (Left-to-Right) → WA

GCC evaluated the left-hand side first:

++t

So t immediately became 1.

Then the right-hand side was evaluated:

pst.update(t, ...)

But now t == 1, meaning the update tried to build the new version from:

roots[1]

instead of:

roots[0]

Since roots[1] had not been created yet, the persistent chain broke completely.

The Fix

Simply separate the increment from the function call:

++t;
pst.roots[t] = pst.update(t - 1, l, r, d);

or:

int newRoot = pst.update(t, l, r, d);
pst.roots[++t] = newRoot;

A Fun C++17 Fact

The C++ standards committee recognized how dangerous and confusing this behavior was.

Starting from C++17, the evaluation order of assignment expressions became well-defined, and the right-hand side is guaranteed to be evaluated before the left-hand side.

So this exact bug disappears in modern C++ standards.

This was one of the sneakiest bugs I’ve encountered, because the code looked perfectly fine and even passed on one compiler.

A good reminder that undefined/unspecified behavior in C++ can destroy your sanity :)