Any number type can be constructed (or assigned) from any fundamental (built-in) arithmetic type, as long as the conversion isn't lossy (for example float to int conversion):

cpp_dec_float_50 df(0.5);   // OK construction from double
cpp_int          i(450);    // OK constructs from signed int
cpp_int          j = 3.14;  // Error, lossy conversion.

A number can be explicitly constructed from an arithmetic type, even when the conversion is lossy:

cpp_int          i(3.14);       // OK explicit conversion
i = static_cast<cpp_int>(3.14)  // OK explicit conversion
i.assign(3.14);                 // OK, explicit assign and avoid a temporary from the cast above
i = 3.14;                       // Error, no implicit assignment operator for lossy conversion.
cpp_int          j = 3.14;      // Error, no implicit constructor for lossy conversion.

A number can be converted to any fundamental (built-in) type, via the convert_to member function:

mpz_int z(2);
int i = z.convert_to<int>(); // sets i to 2

A number can be converted to any fundamental (built-in) type, via an explicit conversion operator: this functionality is only available on compilers supporting C++11's explicit conversion syntax.

mpz_int z(2);
int i = z;                     // Error, implicit conversion not allowed.
int j = static_cast<int>(z);   // OK explicit conversion.

Any number type can be explicitly constructed (or assigned) from a const char* or a std::string:

// pi to 50 places from a string:
cpp_dec_float_50 df("3.14159265358979323846264338327950288419716939937510");
// Integer type will automatically detect "0x" and "0" prefixes and parse the string accordingly:
cpp_int          i("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFF000000000000000");
// Invalid input always results in a std::runtime_error being thrown:
i = static_cast<cpp_int>("3.14");
// implicit conversions from strings are not allowed:
i = "23"; // Error, no assignment operator for implicit conversion from string
// assign member function, avoids having to create a temporary via a static_cast:
i.assign("23");  // OK

Any number type will interoperate with the fundamental (built-in) types in arithmetic expressions as long as the conversions are not lossy:

// pi to 50 places from a string:
cpp_dec_float_50 df = "3.14159265358979323846264338327950288419716939937510";
// Multiply by 2 - using an integer literal here is usually more efficient
// than constructing a temporary:
df *= 2;

// You can't mix integer types with floats though:
cpp_int i = 2;
i *= 3.14;  // Error, no *= operator will be found.

Any number type can be streamed to and from the C++ iostreams:

cpp_dec_float_50 df = "3.14159265358979323846264338327950288419716939937510";
// Now print at full precision:
std::cout << std::setprecision(std::numeric_limits<cpp_dec_float_50>::max_digits10)
   << df << std::endl
cpp_int i = 1;
i <<= 256;
// Now print in hex format with prefix:
std::cout << std::hex << std::showbase << i << std::endl;

Interconversions between number types of the same family are allowed and are implicit conversions if no loss of precision is involved, and explicit if it is:

int128_t     i128 = 0;
int266_t     i256 = i128;  // OK implicit widening conversion
i128_t            = i256;  // Error, no assignment operator found, narrowing conversion is explicit.
i128_t            = static_cast<int128_t>(i256); // OK, explicit narrowing conversion.

mpz_int      z    = 0;
mpf_float    f    = z;    // OK, GMP handles this conversion natively, and it's not lossy and therefore implicit.

mpf_float_50 f50  = 2;
f                 = f50;  // OK, conversion from fixed to variable precision, f will have 50 digits precision.
f50               = f;    // Error, conversion from variable to fixed precision is potentially lossy, explicit cast required.

Some interconversions between number types are completely generic, and are always available, albeit the conversions are always explicit:

cpp_int cppi(2);
// We can always convert between numbers of the same category -
// int to int, rational to rational, or float to float, so this is OK
// as long as we use an explicit conversion:
mpz_int z(cppi);
// We can always promote from int to rational, int to float, or rational to float:
cpp_rational     cppr(cppi);  // OK, int to rational
cpp_dec_float_50 df(cppi);    // OK, int to float
df                  = static_cast<cpp_dec_float_50>(cppr);  // OK, explicit rational to float conversion
// However narrowing and/or implicit conversions always fail:
cppi                =   df;    // Compiler error, conversion not allowed

Other interconversions may be allowed as special cases, whenever the backend allows it:

mpf_t     m;           // Native GMP type.
mpf_init_set_ui(m, 0); // set to a value;
mpf_float i(m);        // copies the value of the native type.