What is the meaning of numeric_limits::digits10

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numeric_limits::digits10 is the number of decimal digits that can be held without loss.

For example numeric_limits<unsigned char>::digits10 is 2. This means that an unsigned char can hold 0..99 without loss. If it were 3 it could hold 0..999, but as we all know it can only hold 0..255.

This manual page has an example for floating point numbers, which (when shortened) shows that

cout << numeric_limits<float>::digits10 <<endl;
float f = (float)99999999; // 8 digits
cout.precision ( 10 );
cout << "The float is; " << f << endl;

prints

6
The float is; 100000000

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