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POUse multiple ofstreams to write to a single output file in c++
 

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  1. POUse multiple ofstreams to write to a single output file in c++
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    <p>I have class <code>Writer</code> that has two <code>ofstream</code> members.<br/> Both streams are associated with the <strong>same</strong> output file. I'd like to use both streams in <code>Writer::write</code> method, but to make sure that each stream writes to the end of the real output file.<br/></p> <p><em><strong>Code</em></strong></p> <pre><code>class my_ofstream1: public ofstream { // implement some functions. // using internal, extended type of streambuf }; class my_ofstream2: public ofstream { // implement some functions. // using internal, extended type of streambuf // (not the same type as found in my_ofstream1) }; class Writer { public: void open(string path) { f1.open(path.c_str(),ios_base::out); f2.open(path.c_str(),ios_base::out); } void close() { f1.close(); f2.close(); } void write() { string s1 = "some string 1"; string s2 = "some string 2"; f1.write(s1.c_str(), s1.size()); // TBD - ensure stream f2 writes to the end of the actual output file assert(f1.tellp() == f2.tellp()); f2.write(s2.c_str(), s2.size()); } private: my_ofstream1 f1; my_ofstream1 f2; }; void main() { Writer w; w.open("some_file.txt"); w.write(); w.close(); } </code></pre> <p><em><strong>Questions</em></strong><br/><br/> How to ensure <code>f2</code> is in sync with <code>f1</code>? meaning, before writing, stream offset of <code>f2</code> must be in sync with stream offset of <code>f1</code> and vice versa?<br/> I can't use function <a href="http://www.cplusplus.com/reference/ios/ios/rdbuf/" rel="nofollow">std::ios::rdbuf</a> since each <code>ofstream</code> uses special derived <code>streambuf</code>. so by using <code>rdbuf()</code> I'll lose the necessary functionality.<br/> I tried using some of the techniques found in <a href="http://www.math.hkbu.edu.hk/parallel/pgi/doc/pgC++_lib/stdlibug/syn_4687.htm" rel="nofollow">Synchronizing Streams</a> topic but could not make it happen.</p>
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    1. COLook into `tie()`. It allows you to couple a stream (input or output) to an output stream, such that the output of the tied stream gets flushed. In my quick reading of `tie()`, though, it appears the synchronization is only one-way. Otherwise, it seems like you might consider an approach that funnels everything through a common class that serializes and writes everything via the true `ofstream`.
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      1. USJoe Z
    2. COIs the "internal, extended type of streambuf" the same type for both streams? If so, just create both streams writing to the same streambuf object.
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      1. POUse multiple ofstreams to write to a single output file in c++
      1. USJerry Coffin
    3. COUnfortunately, they are not of the same type. Of course, they are both of type streambuf but I must have two different objects since each special streambuf manipulate the data in a different way before writing it to the file.
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      1. USidanshmu
 

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