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    <p>After fixing up your program, and with the hint from @user763305, I suppose you have something like this (note: this program compiles, but invokes undefined behavior when run, since the <code>storage</code> vector is left empty):</p> <pre><code>#include &lt;string&gt; #include &lt;vector&gt; class csv_File { public: csv_File(std::string) {} std::string const&amp; access(int,int) const; void modify_element(int column,int row ,std::string value) { storage.at(row).at(column)=value; } private: mutable std::vector &lt; std::vector&lt;std::string&gt; &gt; storage; }; std::string const&amp; csv_File::access(int column,int row) const { return storage.at(row).at(column); } const csv_File&amp; Factory() { static csv_File foo("foo.txt"); return foo; } int main(){ const csv_File&amp; f(Factory()); f.modify_element(1, 2, "hello"); } </code></pre> <p>and you get an error like this:</p> <pre><code>cs.cc: In function ‘int main()’: cs.cc:24: error: passing ‘const csv_File’ as ‘this’ argument of ‘void csv_File::modify_element(int, int, std::string)’ discards qualifiers </code></pre> <p>So, you have a <code>const</code> object and are trying to invoke <code>modify_element</code> on it. Logically, this is a mistake -- you can't modify <code>const</code> objects!</p> <p>You have, as I see it, two choices for a fix. Either you can declare the <code>vector</code> object <code>mutable</code> and the <code>modify_element</code> method <code>const</code>, or you can modify your object factory to return non-const references.</p> <p>Solution 1:</p> <pre><code>... void modify_element(int column,int row ,std::string value) const { storage.at(row).at(column)=value; } ... </code></pre> <p>Solution 2:</p> <pre><code>... const csv_File&amp; Factory() { static csv_File foo("foo.txt"); return foo; } csv_File&amp; RW_Factory() { static csv_File foo("foo.txt"); return foo; } ... const csv_File&amp; f(RW_Factory()); ... </code></pre> <p><hr /> <strong><em>EDIT</em></strong> And, yes, if you previously returned a reference to a <code>string</code> in your <code>vector</code>, and you subsequently assign a new value to that <code>string</code> as you do in <code>modify_element</code>, then the previous reference will reflect the new value.</p> <p>Note that the previous reference will be invalid if you destroy the vector, erase that <code>vector</code> element, or do anything to cause the <code>vector</code> to grow beyond its previous capacity.</p>
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    1. COAside: `modify_element` really should takes its third parameter by reference, not by value: `const std::string& value`.
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    2. COThanks, for taking the time to figure out my incomplete code. The first solution provided essentially does what I am looking for. Can you give some more information why the reference becomes invalid of the vector grows?
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    3. COWhen a `vector` grows, if it grows beyond the capacity of its memory region, it must be copied into a new, larger memory region; the old now-too-small region is destroyed. Thus, any pointer (or reference or iterator) pointing into the old, now discarded memory is invalid. From the C++03 standard, §23.2.4.3, ¶1: "[vector::insert] causes reallocation if the new size is greater than the old capacity", and §23.2.4.2, ¶5: "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence."
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