Why is initializing an integer in C++ to 010 different from initializing it to 10?

When an integer is initialized as int a = 010, a is actually set to 8, but开发者_StackOverflow中文版 for int a = 10, a is set to 10. Can anyone tell me why a is not set to 10 for int a = 010?

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Because it's interpreting 010 as a number in octal format. And in a base-8 system, the number 10 is equal to the number 8 in base-10 (our standard counting system).

More generally, in the world of C++, prefixing an integer literal with 0 specifies an octal literal, so the compiler is behaving exactly as expected.

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0 before the number means it's in octal notation. So since octal uses a base of 8, 010 would equal 8.

In the same way 0x is used for hexadecimal notation which uses the base of 16. So 0x10 would equal 16 in decimal.

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In C, C++, Objective C and related languages a 0 prefix signifies an octal literal constant, so 010 = 8 in decimal.

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Leading 0 in 010 means that this number is in octal form. So 010 means 8 in decimal.