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POWrong version of overridden [] operator called in class
 

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  1. POWrong version of overridden [] operator called in class
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    <p>I have a simple two-dimensional line class which holds two vectors of doubles. I have added getValue and setValue functions, but would prefer the public interface to have the square bracket operator available alongside these functions. The following code shows the implementation and use:</p> <pre><code>#include &lt;vector&gt; #include &lt;algorithm&gt; #include &lt;cassert&gt; class Simple2DLine { public: Simple2DLine(); // Simple read method with linear interpolation double getValue(double x) const; // Simple write method, adds a curve point, keeping the arrays sorted void setValue(double x, double y); double&amp; operator [](double x); const double operator [](double x) const; private: std::vector&lt;double&gt; m_X; std::vector&lt;double&gt; m_Y; int getNearestIndex(double x) const; }; Simple2DLine::Simple2DLine() { } void Simple2DLine::setValue(double x, double y) { // Get the index of the point at or just before 'x' int idx = getNearestIndex(x); // Check if the exact point already exists. if (idx &gt;= 0) { if (m_X[idx] == x) { m_Y[idx] = y; return; } else { // Insert adds the value just BEFORE idx, so increment it before inserting. ++idx; m_X.insert(m_X.begin() + idx,x); m_Y.insert(m_Y.begin() + idx,y); return; } } // Otherwise, just insert at the front. m_X.insert(m_X.begin(),x); m_Y.insert(m_Y.begin(),y); } double Simple2DLine::getValue(double x) const { // Make sure there are points - if not, return 0. if (m_X.size() == 0) { return 0; } // Make sure it's not out of bounds. if (x &lt; m_X.front() || x &gt; m_X.back()) { return 0; } // Check if it's at or after the last point if (x == m_X.back()) { return m_X.back(); } // Find the point just before the given point. int idx = getNearestIndex(x); // Check if we're on the exact point if (m_X[idx] == x) { return m_X[idx]; } else { // Find the distance from the nearest point and linearly interpolate. double dist = x - m_X[idx]; return m_Y[idx] + dist * (m_Y[idx + 1] - m_Y[idx]) / (m_X[idx + 1] - m_X[idx]); } } double&amp; Simple2DLine::operator [](double x) { // Create a space for the new value setValue(x,0.0); int idx = getNearestIndex(x); return m_Y[idx]; } const double Simple2DLine::operator [](double x) const { return getValue(x); } // Returns the index of the point at or just before 'x'. Invalid values return -1. int Simple2DLine::getNearestIndex(double x) const { if (m_X.empty()) { return -1; } std::vector&lt;double&gt;::const_iterator xBegin(m_X.begin()); std::vector&lt;double&gt;::const_iterator xEnd(m_X.end()); // Get an iterator to the first value GREATER than our search value std::vector&lt;double&gt;::const_iterator it = upper_bound(xBegin,xEnd,x); // If the iterator is at the beginning, all values are greater if (it == xBegin) { return -1; } // Otherwise, decrement the iterator by 1, and return its' distance from the start. return (it - 1) - xBegin; } int main(int argc, char** argv) { Simple2DLine tda; tda.setValue(0.0,10.0); tda.setValue(1.0,15.0); tda.setValue(2.0,20.0); tda.setValue(3.0,25.0); double tmp = tda.getValue(0.5); assert(abs(tmp - 12.5) &lt; 0.000001); tmp = tda.getValue(1.5); assert(abs(tmp - 17.5) &lt; 0.000001); tmp = tda.getValue(2.5); assert(abs(tmp - 22.5) &lt; 0.000001); // Here, the wrong version of the overridden operator is being called. tmp = tda[1.5]; tda[2.5] = 22.5; } </code></pre> <p>When I access the line object in the following fashion, the correct version of the operator is called (non-const)</p> <pre><code>tda[2.5] = 22.5; </code></pre> <p>However, when I try to use the const version, as follows:</p> <pre><code>tmp = tda[1.5]; </code></pre> <p>the non-const version is called. Is there an error in my implementation? Or is it not possible to access the class in this fashion?</p>
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