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POC++ vector of strings, why assignment results in same char references?
 

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  1. POC++ vector of strings, why assignment results in same char references?
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    <p>I am currently learning C++, and I wanted to manually input values to a vector of strings, processing each character before storing it. This is the code I used:</p> <pre><code>#include &lt;iostream&gt; #include &lt;string&gt; #include &lt;vector&gt; using namespace std; int main() { int i, height = 5, width = 5; vector&lt;string&gt; lab; lab.resize(height, ""); string s; //For input reference cout &lt;&lt; endl &lt;&lt; "Input data:" &lt;&lt; endl &lt;&lt; "01234" &lt;&lt; endl; //Input values for (i = 0; i &lt; height; i++) { getline (cin, s); for (int j = 0; j &lt; width; j++) { //Process char [...] lab[i][j] = s.at(j); } } //Show Matrix cout &lt;&lt; endl &lt;&lt; endl; for (int i = 0; i &lt; height; i++) { for (int j = 0; j &lt; width; j++) { cout &lt;&lt; lab[i][j] &lt;&lt; " "; } cout &lt;&lt; endl; } //Show Addresses cout &lt;&lt; endl &lt;&lt; endl; for (int i = 0; i &lt; height; i++) { cout &lt;&lt; "String Addr: " &lt;&lt; (void*) &amp;lab[i] &lt;&lt; " | Chars Addr: "; for (int j = 0; j &lt; width; j++) { //cout &lt;&lt; lab[i][j] &lt;&lt; " "; cout &lt;&lt; (void*) &amp;lab[i][j] &lt;&lt; " "; } cout &lt;&lt; endl; } cout &lt;&lt; endl; return 0; } </code></pre> <p>The problem was that the characters inside the strings have the same address (I don't know why). Here is an example of one execution of the program:</p> <pre><code>Input data: 01234 aaaaa bbbbb ccccc ddddd abcde a b c d e a b c d e a b c d e a b c d e a b c d e String Addr: 0x8fe8008 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610 String Addr: 0x8fe800c | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610 String Addr: 0x8fe8010 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610 String Addr: 0x8fe8014 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610 String Addr: 0x8fe8018 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610 </code></pre> <p>At the end you can see that the addresses of the chars are the same, while the ones for the strings are different. The final matrix in the example should have been:</p> <pre><code>a a a a a b b b b b c c c c c d d d d d a b c d e </code></pre> <p>But instead, it is:</p> <pre><code>a b c d e a b c d e a b c d e a b c d e a b c d e </code></pre> <p>While I already solved the problem, using the <code>+=</code> operator instead of assigning using <code>[]</code>, I still don't know exactly what hapened with the code above. Why does the chars have the same reference?</p>
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    1. CObefore the line `lab[i][j] = s.at(j);` do you resize `lab[i]` at all?
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      1. USSirGuy
    2. COHint: Replace `lab[i][j]` with `lab[i].at(j)` and you should get an `out_of_range` exception
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    3. CO@gx_ Yes, but that's really not what you want. If his type were `std::vector<std::vector<char> >`, he'd get error messages at runtime, with an abort (assuming the correct compiler flags), but for some reason, the compilers I have access to don't do this with `std::string`. (I'd consider this a bug.)
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      1. USJames Kanze
 

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