CODEWITHSUNDEEP
c++type-conversioncompiler-warningssignedtruncation
sayanel
sayanel

Reputation: 389

Implicit conversion warning int to int-lookalike

My compiler warms me about a lot of implicit conversion:

Some I do understand, like

implicit conversion changes signedness: 'int' to 'std::vector::size_type' (aka 'unsigned long')`

when I do myvector.resize(myInt) .

Other more obscure, like

implicit conversion loses integer precision: 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::size_type' (aka 'unsigned long') to 'int'

when I do myInt=myString.size(), or

implicit conversion changes signedness: 'int' to 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::size_type' (aka 'unsigned long')

when I call myString[myInt]

I all cases, I do understand why I get these message (int-uint, etc), however their use is non-ambiguous in the program. What do I risk if I do not hunt change the type of my variables to delete these warning? My previous compiler didn't warn me of anything, so when I changed my PC I suddenly had dozens of warning.

Upvotes: 3

Views: 12087

Answers (2)

L. F.
L. F.

Reputation: 20559

I all cases, I do understand why I get these message (int-uint, etc), however their use is non-ambiguous in the program. What do I risk if I do not hunt change the type of my variables to delete these warning?

It's not about ambiguity. If you convert a signed value to an unsigned type, you may get bizarre results:

long i = -2147483645;        // for example

std::vector<int> vec{0, 1, 2, 3, 4};
std::cout << vec[i] << "\n"; // undefined behavior, not always caught

Here, assuming a 32-bit system, i gets converted to 2147483651 before it is passed to operator[], causing undefined behavior.

If you are really sure the conversion is harmless, you can do a static_cast:

static_cast<std::vector<int>::size_type>(i)

My previous compiler didn't warn me of anything, so when I changed my PC I suddenly had dozens of warning.

Always turn on warnings. At least do -Wall. When writing libraries, I tend to make sure my code compiles cleanly even with -Wextra. They can help you catch a lot of bugs.

Upvotes: 5

IIRistoII
IIRistoII

Reputation: 330

The size_type of the stl boils down to an unsigned long for your compiler. That issues the implicit conversion changes signedness warning when assigning it to a signed type like int.

Additionally the width of a long depends on the cpu architecture, i.e. it 32 Bit on x86 but 64 Bit on x64, whereas an int is defined to 32 Bit width. This issues the implicit conversion loses integer precision warning.

My previous compiler didn't warn me of anything, so when I changed my PC I suddenly had dozens of warning

Probably the warning level on your new PC is higher. Check thecompiler options for -W3 (Windows), -Wall or -Wextra (GCC).

What do I risk if I do not hunt change the type of my variables to delete these warning?

It's at least bad style and causes undefined behaviour if the size of the stl container does not fit into an int.

Upvotes: 2

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