C/C++: What does this syntax mean?

Question

What does sampler2D texture0 : DEFINESAMPLER(0); do? It's taken from a header inside a game's directory. I'm confused about the use of the colon.

---

Edit: More context, as requested.

There really isn't much more context to mention. The line is from a file resources/Shaders/ShaderMacros.h from a game called Super Meat Boy (the Steam version, at least). The header just contains a bunch of declarations such as the one I mentioned above:

/**
Defines for profile specific compiling 
*/
#define DEFINEFLOATCONSTANT(x) register(c##x)
#define HardwareInstanceTrans float4x4 matInstanceTransform : TEXCOORD1
#define HardwareInstanceTexCoordOffset float4 vInstanceTexCoordOffset : TEXCOORD5

#ifdef SHADER_DX
    #define matrixmul(mat, pos) mul(pos, mat)
#else
    #define matrixmul(mat, pos) mul(mat, pos)
#endif


#define DEFINESAMPLER(x) register(s##x)

/*************************************************************
SAMPLERS
*************************************************************/
sampler2D texture0 : DEFINESAMPLER(0);
sampler2D texture1 : DEFINESAMPLER(1);
sampler2D texture2 : DEFINESAMPLER(2);
sampler2D texture3 : DEFINESAMPLER(3);
sampler2D texture4 : DEFINESAMPLER(4);
sampler2D texture5 : DEFINESAMPLER(5);
sampler2D texture6 : DEFINESAMPLER(6);
sampler2D texture7 : DEFINESAMPLER(7);
sampler2D texture8 : DEFINESAMPLER(8);
sampler2D texture9 : DEFINESAMPLER(9);
sampler2D texture10 : DEFINESAMPLER(10);
sampler2D texture11 : DEFINESAMPLER(11);
sampler2D texture12 : DEFINESAMPLER(12);
sampler2D texture13 : DEFINESAMPLER(13);
sampler2D texture14 : DEFINESAMPLER(14);
sampler2D texture15 : DEFINESAMPLER(15);

/*************************************************************
VERTEX SHADER CONSTANTS
*

************************************************************/
float4x4 matWorld : DEFINEFLOATCONSTANT(0);         //Uses 4 registers

#ifdef SHADER_DX
float3x4 mat3x4TexGen0 : DEFINEFLOATCONSTANT(4);    //Uses 3 registers
float3x4 mat3x4TexGen1 : DEFINEFLOATCONSTANT(7);    //Uses 3 registers
float3x4 mat3x4TexGen2 : DEFINEFLOATCONSTANT(10);   //Uses 3 registers
float3x4 mat3x4TexGen3 : DEFINEFLOATCONSTANT(13);   //Uses 3 registers
float3x4 mat3x4TexGen4 : DEFINEFLOATCONSTANT(16);   //Uses 3 registers
float3x4 mat3x4TexGen5 : DEFINEFLOATCONSTANT(19);   //Uses 3 registers
float3x4 mat3x4TexGen6 : DEFINEFLOATCONSTANT(22);   //Uses 3 registers
float3x4 mat3x4TexGen7 : DEFINEFLOATCONSTANT(25);   //Uses 3 registers

float3x4 mat2x4TexGen0 : DEFINEFLOATCONSTANT(28);   //Uses 2 registers
float3x4 mat2x4TexGen1 : DEFINEFLOATCONSTANT(30);   //Uses 2 registers
float3x4 mat2x4TexGen2 : DEFINEFLOATCONSTANT(32);   //Uses 2 registers
float3x4 mat2x4TexGen3 : DEFINEFLOATCONSTANT(34);   //Uses 2 registers
float3x4 mat2x4TexGen4 : DEFINEFLOATCONSTANT(36);   //Uses 2 registers
float3x4 mat2x4TexGen5 : DEFINEFLOATCONSTANT(38);   //Uses 2 registers
float3x4 mat2x4TexGen6 : DEFINEFLOATCONSTANT(40);   //Uses 2 registers
float3x4 mat2x4TexGen7 : DEFINEFLOATCONSTANT(42);   //Uses 2 registers
#else

/** Texture coord generation matricies */
float4x3 mat3x4TexGen0 : DEFINEFLOATCONSTANT(4);    //Uses 3 registers
float4x3 mat3x4TexGen1 : DEFINEFLOATCONSTANT(7);    //Uses 3 registers
float4x3 mat3x4TexGen2 : DEFINEFLOATCONSTANT(10);   //Uses 3 registers
float4x3 mat3x4TexGen3 : DEFINEFLOATCONSTANT(13);   //Uses 3 registers
float4x3 mat3x4TexGen4 : DEFINEFLOATCONSTANT(16);   //Uses 3 registers
float4x3 mat3x4TexGen5 : DEFINEFLOATCONSTANT(19);   //Uses 3 registers
float4x3 mat3x4TexGen6 : DEFINEFLOATCONSTANT(22);   //Uses 3 registers
float4x3 mat3x4TexGen7 : DEFINEFLOATCONSTANT(25);   //Uses 3 registers

float4x3 mat2x4TexGen0 : DEFINEFLOATCONSTANT(28);   //Uses 2 registers
float4x3 mat2x4TexGen1 : DEFINEFLOATCONSTANT(30);   //Uses 2 registers
float4x3 mat2x4TexGen2 : DEFINEFLOATCONSTANT(32);   //Uses 2 registers
float4x3 mat2x4TexGen3 : DEFINEFLOATCONSTANT(34);   //Uses 2 registers
float4x3 mat2x4TexGen4 : DEFINEFLOATCONSTANT(36);   //Uses 2 registers
float4x3 mat2x4TexGen5 : DEFINEFLOATCONSTANT(38);   //Uses 2 registers
float4x3 mat2x4TexGen6 : DEFINEFLOATCONSTANT(40);   //Uses 2 registers
float4x3 mat2x4TexGen7 : DEFINEFLOATCONSTANT(42);   //Uses 2 registers

#endif

/***
Graphics Pipeline stuff
*/

//ColorConstant1
float4 cColorConstant1 : DEFINEFLOATCONSTANT(0);    //uses 1 register
float4 cColorConstant2 : DEFINEFLOATCONSTANT(1);    //uses 1 register
float4 cColorConstant3 : DEFINEFLOATCONSTANT(2);    //uses 1 register

float4 vShaderParams : DEFINEFLOATCONSTANT(3); //uses 1 register

float4 cFogColor : DEFINEFLOATCONSTANT(4); //uses 1 register
float4 vFogParams : DEFINEFLOATCONSTANT(5);

#define FOGMAXDIST vFogParams.x //this parameter is CPU calculated as 1 / elementZ. This is only applicable for 2D
#define FOGITEMDIST vFogParams.y //item distance set by the CPU

Answer 1

This isn't a C or C++ header file. This is a HLSL shader file. In short, the sampler2D texture0 : DEFINESAMPLER(0); statement creates a texture input into the shader. For more information about HLSL see MSDN HLSL Documentation.

Answer 2

Seems like that headerfile gets preprocessed and then gets included in the actual shader files.

sampler2D texture0 : DEFINESAMPLER(0);

For example expands to

sampler2D texture0 : register(s0);

after preprocessing. That texture sampler then gets assigned to a sampler register 0. Look at this msdn site for more information.