Warning in C++: "warning: statement has no effect" in for loop initialization

Question

I have the following code:

void CScriptTable::EnumReferences(asIScriptEngine *engine)
{
    if (m_table)
    {
        // Call the gc enum callback for each nested table
        size_t col = 0, row = 0, num_cols = m_table->numCols(),
               num_rows = m_table->numRows();

        for(col; col < num_cols; col++)   // Line 92
        {
            if (m_table->getColType(col) == COL_TABLE)
            {
                for (row; row < num_rows; row++)   // Line 95
                {
                    Table * tbl = m_table->getTable(row, col);
                    engine->GCEnumCallback(tbl);
                }
            }
        }
    }
}

When compiling, (g++), the warning ("statement has no effect") is issued for line 92 & 95 (indicated in the snippet above).

I can't see why they don't have any effect, even though I have been staring at it for a while. What am I missing?

Split up your variable declarations. That line is insane. And why did you declare your loop variables outside the loop anyway? Don’t do that. — Konrad Rudolph
– Konrad Rudolph, Commented Oct 31, 2011 at 9:21
There is a subtle bug in your code. row is set at 0 once, before entering the col loop, and never reset afterwards. Therefore, on the first iteration of the outer (col) loop, you do iterate on the rows, but for all the other iterations, you just do nothing. See @b.buchhold's answer which solve both your original question and this bug... and learn to declare your variables in as tight a scope as possible. — Matthieu M.
– Matthieu M., Commented Oct 31, 2011 at 9:58

Lekensteyn · Accepted Answer · 2011-10-31 09:15:30Z

8

It's about the col and row part in the initializer part of the loops. Those statements do nothing. Just remove it:

for( ; col < num_cols; col++)

answered Oct 31, 2011 at 9:15

Lekensteyn

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Comments

Peter Mortensen · Accepted Answer · 2025-08-22 11:44:50Z

I guess you want to iterate over all columns and for each of them over all rows. So better change your code to something like this:

The first statement in the for loop is executed once, i.e., at the time the loop is entered at first. Since you want to include row number zero for all further columns, you have to set row to 0 for each column:

void CScriptTable::EnumReferences(asIScriptEngine *engine)
{
    if (m_table)
    {
        // Call the gc enum callback for each nested table
        size_t num_cols = m_table->numCols(), num_rows = m_table->numRows();

        for(size_t col = 0; col < num_cols; col++ )   // Line 92
        {
            if (m_table->getColType(col) == COL_TABLE) {
                for (size_t row = 0; row < num_rows; row++){  // Line 95
                    Table * tbl = m_table->getTable(row, col);
                    engine->GCEnumCallback(tbl);
                }
            }
        }
    }
}

FailedDev · Accepted Answer · 2011-10-31 09:15:00Z

5

for( col; col < num_cols; col++ )

col;

This has no effect. You have to assign something to it, or don't write it at all. Since you assign it out of your loop you just need to leave an empty; at that place or use while loops.

answered Oct 31, 2011 at 9:15

FailedDev

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Comments

Peter Mortensen · Accepted Answer · 2025-08-22 11:45:47Z

You're getting these warnings, because the initialization statements in the for loops are expressions that do nothing:

for(col; col < num_cols; col++)  // line 92: "col" has no effect
for(row; row < num_rows; row++)  // line 95: "row" has no effect

Since you've already initialized these variables outside the loop, you might want to omit them from the for statement:

for(; col < num_cols; col++)  // line 92
for(; row < num_rows; row++)  // line 95

However, the best thing to do here is to initialize the variables in the for loops themselves rather than outside them:

// Call the gc enum callback for each nested table
size_t num_cols = m_table->numCols(), num_rows = m_table->numRows();

for(size_t col = 0; col < num_cols; col++ )   // Line 92
{
    if (m_table->getColType(col) == COL_TABLE) {
        for (size_t row = 0; row < num_rows; row++) {  // Line 95
            Table * tbl = m_table->getTable(row, col);
            engine->GCEnumCallback(tbl);
        }
    }
}

Vladimir · Accepted Answer · 2011-10-31 09:16:04Z

0

My guess is you could simply use

for(; col < num_cols; col++ )   // Line 92

and

for (; row < num_rows; row++) {  // Line 95

answered Oct 31, 2011 at 9:16

Vladimir

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Comments

Peter Mortensen · Accepted Answer · 2025-08-22 11:39:09Z

0

If you really want to type the variable names in your for loop (as a tip maybe?), try by changing

for(col; col < num_cols; col++)

to something like this

for(col = col; col < num_cols; col++)

for both the lines. It should do the job.

edited Aug 22 at 11:39

Peter Mortensen

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answered Oct 31, 2011 at 9:14

moongoal

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2 Comments

Potatoswatter Over a year ago

That has the potential to hide an uninitialized variable warning, or generate a particularly awful "variable initialized with itself" warning. I've even seen that crash a fragile compiler. And it's bad style.

moongoal Over a year ago

That error happens when 'col' has not yet been initialized, which is not the case.. I agree with you, it's really bad style, but I'm assuming he knows the right syntax..

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