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    <p>You are translating the ASCII into bits using the constructor of <code>bitset</code>. That causes your decoded bits to be in the <code>bitset</code> object rather than the <code>union</code>. To get raw bits out of a bitset, use the <code>to_ulong</code> method:</p> <pre><code>#include&lt;climits&gt; #include&lt;iostream&gt; #include&lt;fstream&gt; #include&lt;bitset&gt; int main( int, char** ) { std::ifstream inputFile( "testData.txt", std::ios_base::in | std::ios_base::binary ); if( !inputFile ) std::cout &lt;&lt; "Failed to open input file!" &lt;&lt; std::endl; char buffer[ CHAR_BIT * sizeof(float) ]; inputFile.read( buffer, CHAR_BIT * sizeof(float) ); std::cout &lt;&lt; "cout.write of input from file = "; std::cout.write( buffer, CHAR_BIT * sizeof(float) ); std::cout &lt;&lt; std::endl; union { float f[ sizeof(unsigned long)/sizeof(float) ]; unsigned long l; } funion; funion.l = std::bitset&lt;32&gt;( std::string( buffer ) ).to_ulong(); std::cout &lt;&lt; "funion.f = " &lt;&lt; funion.f[0] &lt;&lt; " bits = " &lt;&lt; std::hex &lt;&lt;funion.l &lt;&lt; std::endl; inputFile.close(); return 0; } </code></pre> <p>This generally assumes that your FPU operates with the same endianness as the integer part of your CPU, and that <code>sizeof(long) &gt;= sizeof(float)</code>… less guaranteed for <code>double</code>, and indeed the trick is harder to make portable for 32-bit machines with 64-bit FPUs.</p> <p><strong>Edit:</strong> now that I've made the members of the union equal sized, I see that this code is sensitive to endianness. The decoded <code>float</code> will be in the last element of the array on a big-endian machine, first element on little-endian. :v( . Maybe the best approach would be to attempt to give the integer member of the union exactly as many bits as the FP member, and perform a narrowing cast after getting <code>to_ulong</code>. Very difficult to maintain the standard of portability you seemed to be shooting for in the original code.</p>
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    1. COI have read you're answer, it does completely fix my problem so thank you very much. I'll mark this as answered but I wanted to see whether Jonathan Leffler was presenting something different before I did. I'd also rather use a solution that had a character array vice a unsigned long int as the other member of the union (because it's not clear how to extend union to any other type than float to me at least but in the char[] case it is trivial).
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    2. CO@bpw: If you use a `char[]` to store a bit-array, each `char` object stores 8 bits. If you use a `long[]` to store a bit-array, each `long` object stores sizeof(long)*8 bits. Unfortunately, `bitset` will only return one `long`'s worth of bits to you, so you are essentially restricted to a `long[1]`. It's really a limit of `bitset` and you would need to code your own to get around it.
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    3. CO@bpw (from other thread): You can jam a single packet into a `union`, at least. All members of a union are guaranteed to have the same address. A series of packets can be a sequence of union pointers into a buffer. Use that initial `int` to determine what kind of object each union really is… this behavior is defined and guaranteed safe by C++ §9.5/1. However, don't forget to perform endian conversion on every member.
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